[ARM] 2909/1: remove IXP2000_PROD_ID

Patch from Lennert Buytenhek

The intel docs call it IXP2000_PRODUCT_ID, and we have a definition
for IXP2000_PRODUCT_ID as well, so IXP2000_PROD_ID can go.  It's only
used in one place.

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>