dts: bcm2711: add "High Peripheral" mode overlay

The following addresses change:

 - 0xfc00_0000 -> 0x4_7c00_0000
 - 0xff80_0000 -> 0x4_c000_0000

The range 0xfc00_0000-0xffff_ffff becomes available as system RAM on
devices with >= 4 GiB of RAM. Firmware should initialize the memory node
appropriately.

Signed-off-by: Hristo Venev <hristo@venev.name>