ARM: kirkwood: add spi0 alias for dreamplug

ARM: kirkwood: add spi0 alias for dreamplug

The conversion to DM_SPI managed to break accessing the environment on
dreamplug. This is because the environment code relies on being to able
to select the SPI device based on the sequence number. Add an alias so
that the spi0 bus gets sequence number 0.

Reported-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: Stefan Roese <sr@denx.de>