cdrom: Add support for Sega Dreamcast GD-ROM.

This patch adds support for the GD-Rom drive, SEGA's proprietary
implementation of an IDE CD Rom for the SEGA Dreamcast. This driver
implements Sega's Packet Interface (SPI) - at least partially. It will
also read disks in SEGA's propreitary GD format.

Unlike previous drivers (which were never in mainline) this uses DMA and
not PIO to read disks. It is a new driver, not a refactoring of old
drivers.

Signed-off by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>