ARM: shmobile: defconfig: Enable RENESAS_WDT_GEN

R-Car Gen2 and RZ/G1 platforms come with a watchdog IP, therefore enable
its driver by default.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>