regulator: tps6586x: add support for SYS rail

regulator: tps6586x: add support for SYS rail

Device have SYS rail which is always ON. It is system
power bus. LDO5 and LDO_RTC get powered through this rail
internally. Add support for this rail and make the
LDO5/LDO_RTC input supply to "sys".
Update document accordingly.

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