[ARM] 5212/1: pxa: fix build error when CPU_PXA310 is not defined

Fix
arch/arm/mach-pxa/pxa300.c:94: error: 'CKEN_MMC3' undeclared here (not in a function)
when building for PXA300.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>