regulator: palmas: Add support for LDO1 regulator to provide 1.8V

Modify palmas_mmc1_poweron_ldo() API to set the voltage based on the
voltage parameter passed as argument instead of always setting it to
3.0V. This allows MMC1 to set the LDO1 regulator voltage to 3.3V or 1.8V.
1.8V is required to add support for UHS mode.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>