sh: maple: Support block reads and writes.

This patch updates the maple bus to support asynchronous block reads
and writes as well as generally improving the quality of the code and
supporting concurrency (all needed to support the Dreamcast visual
memory unit - a driver will also be posted for that).

Changes in the bus driver necessitate some changes in the two maple bus
input drivers that are currently in mainline.

As well as supporting block reads and writes this code clean up removes
some poor handling of locks, uses an atomic status variable to serialise
access to devices and more robusly handles the general performance
problems of the bus.

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