[ARM] pxa: mmc: add 3rd host controller support for pxa310

This patch is to add the third mmc controller support _only_
for pxa310.

On zylonite, the third controller support one slot.

Signed-off-by: Bridge Wu <bridge.wu@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>