sh: Add RSK2+SH7269 board

The RSK2+SH7269 board uses the SH7269 processor. It is often
referred to as just rsk7269. NOR Flash, SDRAM, serial, USB Host and
ethernet are working.

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