spi/sparc: Allow of_register_spi_devices for sparc

The spi support code works on SPARC too. No reason to exclude it from
the party.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>