soc: renesas: ra: add rom_padding workaround for IAR ROM placement

The arbitrary ordering of ILINK may cause the .last_section
to be placed between the gaps of consecutive sections in the ROMABLE
region.

This commit adds a workaround for Renesas RA devices, which have OFS
registers placed in FLASH. Adding a small .rom_padding section at
the beginning of the ROM_REGION stabilizes the placement order and
ensures that all ROM sections remain within the expected region.

Signed-off-by: Thinh Le Cong <thinh.le.xr@bp.renesas.com>
Signed-off-by: The Nguyen <the.nguyen.yf@renesas.com>