[ARM] 2908/1: add register definitions for ixp2000 msf

Patch from Lennert Buytenhek

We already had a mapping for the msf, but we didn't have any
register definitions for it yet.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>