kirkwood_nand: claim MPP pins on the fly

kirkwood_nand: claim MPP pins on the fly

Claim the MPP pins for the NAND flash controller only when it's actually
being used. This allows the pins to be shared with the SPI interface
which already supports an equivalent on-access MPP reconfiguration.

Reviewed-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Scott Wood <oss@buserror.net>
Signed-off-by: Stefan Roese <sr@denx.de>