mfd: max14577: Map charger device to its own of_node

Add a "maxim,max14577-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 max14577 charger driver in next patches to obtain battery
configuration from DTS.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>