regulator: lp873x: Add support for lp873x PMIC regulators

The regulators set consists of 2 BUCKs and 2 LDOs. The output
voltages are configurable and are meant to supply power to the
main processor and other components. The ramp delay is configurable
for both BUCKs.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>