rtc: omap: Fix selecting external osc

rtc: omap: Fix selecting external osc

commit 3984903a2e3906d3def220e688040ce93368200a upstream.

RTC can be clocked from an external 32KHz oscillator, or from the
Peripheral PLL. The RTC has an internal oscillator buffer to support
direct operation with a crystal.

            ----------------------------------------
            |       Device          ---------       |
            |                       |       |       |
            |                       | RTCSS |       |
            |       ---------       |       |       |
    OSC     |<------| RTC   |       |       |       |
            |------>| OSC   |---    |       |       |
            |       --------   |    |       |       |
            |                   ----|clk    |       |
            |       --------   |    |       |       |
            |       | PRCM  |---    |       |       |
            |       --------        --------        |
            ----------------------------------------

The RTC functional clock is sourced by default from the clock derived
from the Peripheral PLL. In order to select source as external osc clk
the following changes needs to be done:
- Enable the RTC OSC (RTC_OSC_REG[4]OSC32K_GZ = 0)
- Enable the clock mux(RTC_OSC_REG[6]K32CLK_EN = 1)
- Select the external clock source (RTC_OSC_REG[3]32KCLK_SEL = 1)

Fixes: 399cf0f63f6f2 ("rtc: omap: Add external clock enabling support")
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>