ARM: OMAP2: Fix SPI driver failure on 2420 when running multi-omap config

SPI driver will do unhandled fault on OMAP2420 if trying to probe
non-existing SPI busses. Register those additional busses runtime only
for cpus having them.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>