[ARM] export __cpu_flush_dcache_page

Now required for libsas:

  Kernel: arch/arm/boot/Image is ready
  Kernel: arch/arm/boot/zImage is ready
  Building modules, stage 2.
  MODPOST 1096 modules
ERROR: "xscale_flush_kern_dcache_page" [drivers/scsi/libsas/libsas.ko] undefined!

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