pmic: pca9450: Add optional SD_VSEL GPIO for LDO5

LDO5 has two separate control registers. LDO5CTRL_L is used if the
input signal SD_VSEL is low and LDO5CTRL_H if it is high.
The current driver implementation only uses LDO5CTRL_H. To make this
work on boards that have SD_VSEL connected to a GPIO, we add support
for specifying an optional GPIO and setting it to high at probe time.

In the future we might also want to add support for boards that have
SD_VSEL set to a fixed low level. In this case we need to change the
driver to be able to use the LDO5CTRL_L register.

This is a port of the same change in the Linux kernel:
8c67a11bae88 ("regulator: pca9450: Add SD_VSEL GPIO for LDO5")

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Tested-by: Fabio Estevam <festevam@denx.de>