mfd: Add DT binding for Maxim MAX77802 IC

The MAX77802 is a chip that contains regulators, 2 32kHz clocks,
a RTC and an I2C interface to program the individual components.

The are already DT bindings for the regulators and clocks and
these reference to a bindings/mfd/max77802.txt file, that didn't
exist, for the details about the PMIC.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>