regulator: lp873x: Add support for populating input supply

In order to have a proper topology of regulators for a platform, each
registering regulator needs to populate supply_name field for identifying
its supply's name. Add supply_name field for lp873x regulators.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-for-MFD-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>