regulator: tps6586x: Prepare supporting fixed regulators

Add the required definitions and macros to allow easily adding fixed
regulators. This required for the TPS658640 that doesn't allow setting
the LDO_RTC output voltage.

Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Mark Brown <broonie@linaro.org>