arm: mvebu: armada-xp-gp.dts: Add spi0 alias

For correct spi bus detection the spi0 alias is needed in the DT.
Otherwise this error will ocurr in U-Boot:

Invalid bus 0 (err=-19)
Failed to initialize SPI flash at 0:0 (error -19)

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dennis Gilmore <dgilmore@redhat.com>
Tested-by: Dennis Gilmore <dgilmore@redhat.com>