regulator: pca9450: Handle hardware with fixed SD_VSEL for LDO5

There are two ways to set the output voltage of the LD05
regulator. First by writing to the voltage selection registers
and second by toggling the SD_VSEL signal.

Usually board designers connect SD_VSEL to the VSELECT signal
controlled by the USDHC controller, but in some cases the
signal is hardwired to a fixed low level (therefore selecting
3.3V as initial value for allowing to boot from the SD card).

In these cases, the voltage is only determined by the value
of the LDO5CTRL_L register. Introduce a property
nxp,sd-vsel-fixed-low to let the driver know that SD_VSEL
is low and there is no GPIO to actually get that
information from dynamically.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
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Changes for v2:
* new patch