arm64: dts: imx8mq-librem5: Run GPU at 0.9V

According to the imx8mq data sheet running VDD_GPU at 0.9V is enough
when not overclocking to 1GHz (which we currently don't do).

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
3 jobs for mrs/etna-v in 71 minutes and 53 seconds
Status Job ID Name Coverage
  Build
passed #248081
librem5
build:aarch64

00:25:50

passed #247895
librem5:arm64
build:deb

00:46:03

failed #247894
librem5
build:aarch64