sh: Fix up r7780rp highlander CF access size.

R7780RP can't do byte-sized accesses to CF, so needs to do word
sized access with low-byte masking. This same problem exists
on older versions of the R2D, with the same workaround having
been implemented in 43f4b8c7
there. Follow that change for the highlander boards.

This does not impact R7780MP or SH7785 based Highlander modules.

If you're unfortunate enough to be stuck with an R7780RP, this
patch is for you!

Signed-off-by: Paul Mundt <lethal@linux-sh.org>