uboot-mediatek: adjust the reserved memory regions

uboot-mediatek: adjust the reserved memory regions

* Increase mt7986 ATF reserved memory to 256 kiB to follow the linux
  kernel and Trusted Firmware-A changes.

* Remove unnecessary reserved memory region for the WiFi hardware. The
  u-boot can not drive the WiFi hardware. Hence we don't need to keep
  this memory region.

* Remove useless node labels and update comments.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19741
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>