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 supply by it. Update document
accordingly.

[swarren: Instantiate the sys regulator from board-harmony-power.c to
 avoid regression.]

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