riscv: dts: Update memory configuration

In the v2022.10 Icicle reference design, the seg registers have been
changed, resulting in a required change to the memory map.
A small 4MB 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 between
reboots of a specific context.

Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rick Chen <rick@andestech.com>