clocksource: sh_cmt: CMSTR and CMCSR register access update

Update hardware register access code for CMSTR and CMCSR
from using sh_cmt_read() and sh_cmt_write() to make use
of 16-bit register access functions such as sh_cmt_read16()
and sh_cmt_write16(). Also update sh_cmt_read() and
sh_cmt_write() now when the special cases are gone.

This patch moves us one step closer to the goal of separating
counter register access functions from control control register
functions.

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>