clocksource: sh_cmt: CMCNT and CMCOR register access update

Break out the CMCNT and CMCOR register access code
into separate 16-bit and 32-bit functions that are
hooked into callbacks at init time. This reduces
the amount of software calculations happening at
runtime.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: John Stultz <john.stultz@linaro.org>
Tested-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>