mfd: Add device-tree binding doc for PMIC MAX77620/MAX20024

The MAXIM PMIC MAX77620 and MAX20024 are power management IC
which supports RTC, GPIO, DCDC/LDO regulators, interrupt,
watchdog etc.

Add DT binding document for the different functionality of
this device.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>