ARM: shmobile: r8a7791 CMT support

Add r8a7791 CMT support via channel 0 of CMT0. At this
point the CMT is used for clock event operation, but in
the future the arch timer will be the main timer and the
CMT will be used for deep sleep wake up only.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
(cherry picked from commit a7663b88280d00359715817620798e99d54d401c)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>