dt-bindings: regulator: pca9450: Document nxp,sd-vsel-fixed-low property for LDO5

This new property can be used for boards which have the SD_VSEL tied
to a fixed low level. The voltage of LDO5 is therefore only controlled
by writing to the LDO5CTRL_L register.

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