ARM: dts: atlas7: add alias name for spi device

spi framework can use alias name of spi device to retrieve the bus id,
so bus id will not be dynamical but statical and it will be easier for
test for a specified spi device with a fixed name like use spidev.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>