mfd: max77693: Map charger device to its own of_node

Add a "maxim,max77693-charger" of_compatible to the mfd_cell so the MFD
child device (the charger) will have its own of_node set. This will be
used by the max77693 charger driver in next patches to obtain battery
configuration from DTS.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>