librem5: Assign gpu clocks IMX8MQ_GPU_PLL_OUT as parent.

Not running at 25Mhz improves performance.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for gpu-clk in 52 minutes and 33 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #21022
librem5
build:aarch64:Image

00:52:33

passed #21023
librem5
build:aarch64:deb

00:39:30