The reserved-memory node 'wmcpu-reserved@50000000' only applies to
linux kernel and is useless in u-boot.
Remove it in *-u-boot.dtsi to make this memory region usable.
Fixes: 2d6962e0618 (arm: mediatek: add support for MediaTek MT7987 SoC) Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>