arm64: dts: imx8mn: remove GPU power domain reset

The PGC (power gating controller) already handles the reset for the
GPUMIX power domain. By specifying it within the device tree the reset
it issued a 2nd time. This confuses the hardware during power up and
sporadically hangs the SoC. Fix this by removing the reset property and
let the hardware handle the reset.

Fixes: 9a0f3b15 ("arm64: dts: imx8mn: Enable GPU")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>