ARM: shmobile: Restrict TWD support to SoCs that have it

Currently support for the ARM Timer and Watchdog Unit is included
unconditionally, while only some Renesas multicore Cortex-A9 SoCs have
a TWD.

This decreases kernel image size by ca. 2 KiB on SoCs without a TWD.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>