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>

soc/andestech/ae350/linker.ld

@@ -116,7 +116,7 @@ SECTIONS
     {
 		. = ALIGN(16);
 		MPU_ALIGN(__rom_region_size);
-		__rom_region_start = .;
+		__rom_region_start = ABSOLUTE(.);
 /* Located in generated directory. This file is populated by calling
  * zephyr_linker_sources(ROM_START ...).
  */