[ARM] 3620/2: ixp23xx: add uengine loader support

Patch from Lennert Buytenhek

This patch allows the ixp2000 uengine loader that is already in the
tree to also be used on the ixp23xx.

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