sh: Add maple bus support for the SEGA Dreamcast.

The Maple bus is SEGA's proprietary serial bus for peripherals
(keyboard, mouse, controller etc). The bus is capable of some
(limited) hotplugging and operates at up to 2 M/bits.

Drivers of one sort or another existed/exist for 2.4 and a rudimentary
port, which didn't support the 2.6 device driver model was also in
existence.

This driver - for the bus logic itself and for the keyboard (other
drivers will follow) are based on the code and concepts of those old
drivers but have lots of completely rewritten parts.

I have the maple bus code as a built in now as that seems the sane and
rational way to handle something like that - you either want the bus
or you don't.

Signed-off-by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>