regulator: pca9450: Fix control register for LDO5

For LDO5 we need to be able to check the status of the SD_VSEL input in
order to know which control register is used. Read the status of the
SD_VSEL signal via GPIO and use the correct register accordingly.

To use this, the LDO5 node in the devicetree needs the sd-vsel-gpios
property to reference the GPIO that is used to read back the SD_VSEL
status internally. Please note that the SION bit in the IOMUX must be
set if the signal is muxed as VSELECT and controlled by the USDHC
controller.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
---
Changes for v2:
* rebase to current master and stop using generic helper from
  regulator core