dts: bindings: mark power gpio in bg95 as optional

The Quectel BG95 binding currently marks mdm-power-gpios as
required.  This fits designs that expose the PWRKEY pin, but the
BG95-M3 in Mini PCIe form-factor boots automatically via its
on-module “Automatic Power-On Circuit” and does not route
PWRKEY to the card edge.

Signed-off-by: Van Petrosyan <van.petrosyan@sensirion.com>