ARM: shmobile: sh73a0: add a TWD clock

Add a TWD clock on sh73a0 for the smp_twd driver to properly update the
clock's frequency upon cpufreq events.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>