ARM: kirkwood: SBx81LIFKW: disable KIRKWOOD_GPIO

DM_GPIO was already enabled so the MVEBU_GPIO was already available.
Having updated the board code to use the DM_GPIO APIs the KIRKWOOD_GPIO
driver became unnecessary. Disable it for SBx81LIFKW.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>