soc: andestech: linker.ld: fix incorrect padding of rom_mpu_padding

The rom_mpu_padding section is incorrect NAPOT padding for the address of
__rodata_region_end when ROM_BASE is not 0x0, because __rom_region_start
is set to the offset of rom_start section.

Fixed this by use "__rom_region_start = ABSOLUTE(.);" to keep both
__rodata_region_end and __rom_region_start are absolute address.

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