clocksource: sh_cmt: Introduce per-register functions

Introduce sh_cmt_read_cmstr/cmcsr/cmcnt() and
sh_cmt_write_cmstr/cmcsr/cmcnt/cmcor() to in the
future allow us to split counter registers from
control registers and reduce code complexity by
removing sh_cmt_read() and sh_cmt_write().

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>