regulator: tps65910: set input_supply on desc unconditionally

Set the supply_name in the regulator descriptor unconditionally
and make this parameter as required parameter in the device
node for successfully registration of the regulator.

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