gpio: ws16c48: Migrate to the regmap API

gpio: ws16c48: Migrate to the regmap API

The regmap API supports IO port accessors so we can take advantage of
regmap abstractions rather than handling access to the device registers
directly in the driver.

The WinSystems WS16C48 provides the following registers:

    Offset 0x0-0x5: Port 0-5 I/O
    Offset 0x6: Int_Pending
    Offset 0x7: Page/Lock
    Offset 0x8-0xA (Page 1): Pol_0-Pol_2
    Offset 0x8-0xA (Page 2): Enab_0-Enab_2
    Offset 0x8-0xA (Page 3): Int_ID0-Int_ID2

Port 0-5 I/O provides access to 48 lines of digital I/O across six
registers, each bit position corresponding to the respective line.
Writing a 1 to a respective bit position causes that output pin to sink
current, while writing a 0 to the same bit position causes that output
pin to go to a high-impedance state and allows it to be used an input.
Reads on a port report the inverted state (0 = high, 1 = low) of an I/O
pin when used in input mode. Interrupts are supported on Port 0-2.

Int_Pending is a read-only register that reports the combined state of
the INT_ID0 through INT_ID2 registers; an interrupt pending is indicated
when any of the low three bits are set.

The Page/Lock register provides the following bits:

    Bit 0-5: Port 0-5 I/O Lock
    Bit 6-7: Page 0-3 Selection

For Bits 0-5, writing a 1 to a respective bit position locks the output
state of the corresponding I/O port. Writing the page number to Bits 6-7
selects that respective register page for use.

Pol_0-Pol_2 are accessible when Page 1 is selected. Writing a 1 to a
respective bit position selects the rising edge detection interrupts for
that input line, while writing a 0 to the same bit position selects the
falling edge detection interrupts.

Enab_0-Enab_2 are accessible when Page 2 is selected. Writing a 1 to a
respective bit position enables interrupts for that input line, while
writing a 0 to that same bit position clears and disables interrupts for
that input line.

Int_ID0-Int_ID2 are accessible when Page 3 is selected. A respective bit
when read as a 1 indicates that an edge of the polarity set in the
corresponding polarity register was detected for the corresponding input
line. Writing any value to this register clears all pending interrupts
for the register.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/f59de81e80f7198bcfa9a15615c459c38b5d0e08.1680708357.git.william.gray@linaro.org/
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>