sh: clkfwk: Support variable size accesses for MSTP clocks.

The bulk of the MSTP users require 32-bit access, but this isn't the case
for some of the SH-2A parts, so add in some basic infrastructure to let
the CPU define its required access size in preparation.

Requested-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>