serial: sh: Add support DL and CKS register for R8A7794

R8A7794 has DL and CKS register, and these registers are used in external clock
mode. This adds support these for R8A7794.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>