arm64: dts: freescale: Fix VPU G2 clock

Set VPU G2 clock to 300MHz like described in documentation.
This fixes pixels error occurring with large resolution ( >= 2560x1600)
HEVC test stream when using the postprocessor to produce NV12.

Fixes: 4ac7e4a8

 ("arm64: dts: imx8mq: Enable both G1 and G2 VPU's with vpu-blk-ctrl")
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>