ARM: shmobile: r8a7779: Add TWD device to DTS

Now when r8a7779 CCF is in place we can hook up
the ARM Cortex-A9 TWD timer via DTS.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>