soc: andestech: linker.ld: clarify usage of __rom_region_end/size

Clarify usage of __rom_region_end/size in XIP system.
When PMP is enabled, __rom_region_end should be padded to meet the
requirement of PMP entry, and the actual ROM region usage ends at
.last_section instead of __rom_region_end.

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>