serial: sh-sci: Fix up SH-2A SCIF support.

This fixes up support for SH-2(A) SCIFs by introducing a new regtype. As
expected, it's close to the SH-4A SCIF with fifodata, but still different
enough to warrant its own type.

Fixes up a number of FIFO overflows and similar for both SH7203/SH7264.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Tested-by: Federico Fuga <fuga@studiofuga.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>