gpio: Add gpio-fsm driver

gpio: Add gpio-fsm driver

The gpio-fsm driver implements simple state machines that allow GPIOs
to be controlled in response to inputs from other GPIOs - real and
soft/virtual - and time delays. It can:
+ create dummy GPIOs for drivers that demand them,
+ drive multiple GPIOs from a single input, with optional delays,
+ add a debounce circuit to an input,
+ drive pattern sequences onto LEDs
etc.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
gpio-fsm: Fix a build warning

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
gpio-fsm: Rename 'num-soft-gpios' to avoid warning

As of 5.10, the Device Tree parser warns about properties that look
like references to "suppliers" of various services. "num-soft-gpios"
resembles a declaration of a GPIO called "num-soft", causing the value
to be interpreted as a phandle, the owner of which is checked for a
"#gpio-cells" property.

To avoid this warning, rename the gpio-fsm property to "num-swgpios".

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
gpio-fsm: Show state info in /sys/class/gpio-fsm

Add gpio-fsm sysfs entries under /sys/class/gpio-fsm. For each state
machine show the current state, which state (if any) will be entered
after a delay, and the current value of that delay.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
gpio-fsm: Fix shutdown timeout handling

The driver is intended to jump directly to a shutdown state in the
event of a timeout during shutdown, but the sense of the test was
inverted.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
gpio-fsm: Clamp the delay time to zero

The sysfs delay_ms value is calculated live, and it is possible for
the time left to appear to be negative briefly if the timer handling
hasn't completed. Ensure the displayed value never goes below zero,
for the sake of appearances.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>