mfd: Add support for enabling tps65910 external 32-kHz oscillator

Add flag to platform data to enable external 32-kHz crystal oscillator
(or square wave) input.

The tps6591x can use either an internal 32-kHz RC oscillator or an
external crystal (or square wave) to generate the 32-kHz clock.

The default setting depends on the selected boot mode. In boot mode 00
the internal RC oscillator is used at power-on, but the external crystal
oscillator (or square wave) can be enabled by clearing the ck32k_ctrl
flag in the device control register.

Note that there is no way to switch from the external crystal oscillator
to the internal RC oscillator.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>