mfd: palmas: Add support for external control configuration

Some of Palmas resources like clock, SMPSs, LDOs etc can be controlled
by external pins ENABLE1, ENABLE2 or NSLEEP.

Add support to configure these resources to externally controlled.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>