regulator: tps65910: Sleep off rails when ext sleep configured

Keep the rails OFF in sleep mode only when the rails are
controlled by external sleep control.
The devices tps65910 and tps65911, both has the sleep input.
The tps65911's sleep input is not same as tps65910's EN3 and hence
taking care of SLEEP input as separate external sleep control input.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>