sh: Add SH7785 Highlander board support (R7785RP).

This adds preliminary support for the SH7785-based Highlander board.
Some of the Highlander support code is reordered so that most of it
can be reused directly.

This also plugs in missing SH7785 checks in the places that need it,
as this is the first board to support the CPU.

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