Add a low level disk drive access driver ...

This is a low level disk drive communication driver. It uses the real mode
BIOS found on most x86 platforms, to read and write sectors. Used by the
generic disk driver.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>