riscv: dts: microchip: update memory configuration for v2022.10

In the v2022.10 reference design, the seg registers are going to be
changed, resulting in a required change to the memory map in Linux.
A small 4M reservation is made at the end of 32-bit DDR to provide some
memory for the HSS to use, so that it can cache its payload.bin between
reboots of a specific context.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>