ARM: shmobile: defconfig: Enable RZ/A watchdog support

Enable support for the RZ/A watchdog timer, which is used on RZ/A1H and
RZ/A2M boards.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200624131931.3686-5-geert+renesas@glider.be