riscv: linker: fix __rom_region_end in non-XIP system

In non-XIP system, because ROMABLE_REGION == RAMABLE_REGION, setting
__rom_region_size/end symbol at linker script end will mistakely
contain RAMABLE_REGION in it.

Move __rom_region_end symbol to end of common ROMABLE_REGION (text
and rodata) instead in non-XIP system.

Signed-off-by: Jim Shu <cwshu@andestech.com>