regulator: palmas: add support for external control of rails

Palmas rails like LDOs, SMPSs, REGENs, SYSENs can be enable and disable
by register programming through I2C communication as well as it can be
enable/disable with the external control input ENABLE1, ENABLE2 and NSLEEP.

Add support for configuring these rails to be controlled by external control
inputs. This is require to configure the rail's control register as well as
configuration of resource register.

Provide the external input names through parameter "roof-floor". Updated the
DT binding document to details different value of the roof-floor.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>