ARM: shmobile: Add r8a7790 CMT00 clock event

Add clock event support for CMT0 timer channel 0
to the r8a7790 SoC code. On most ARM mach-shmobile
the CMT is hooked up to a 32KHz clock but on r8a7790
a 31.7KHz clock is instead used.

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