sh: Add RSK2+SH7264 board

The RSK2+SH7264 board uses the sh7264 processor. It is often
referred to as just rsk7264. 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>