[ARM] Kirkwood: configure NAND mpp's for db88f6281

In case the board is configured to boot from spi flash, the mpps
will not be configured to select the NAND I/Os. This patch makes
sure to select the NAND I/O's regardless to the boot device type.

Signed-off-by: Saeed Bishara <saeed@marvell.com>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>