riscv: ax25-ae350: Use generic memory size setup

To get memory size from device tree instead of
get_ram_size(). This can avoid memory access fault
in U-Boot proper after PMP configurations in OpenSBI.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>