spi: kirkwood_spi.c: Compile MPP (pin-mux) only for kirkwood SoC's

Compile the pin multiplexing only on Kirkwood platforms. As the
Armada XP doesn't need it.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Tested-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>