power: pmic/regulator: Add basic support for TPS65910

power: pmic/regulator: Add basic support for TPS65910

Texas Instrument's TPS65910 PMIC contains 3 buck DC-DC converts, one
boost DC-DC converter and 8 LDOs. This patch implements driver model
support for the TPS65910 PMIC and its regulators making the get/set
API for regulator value/enable available.
This patch depends on the patch "am33xx: Add a function to query MPU
voltage in uV" to build correctly. For boards relying on the DT
include file tps65910.dtsi the v3 patch "power: extend prefix match
to regulator-name property" and an appropriate regulator naming is
also required.

Signed-off-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Simon Glass <sjg@chromium.org>