soc: riscv: andes_v5: fix PMA compilation warnings

These static functions in the `pma.c` are used only when
`CONFIG_NOCACHE_MEMORY` is enabled, so guard them accordingly.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>