of/spi: Fix OF-style driver binding of spi devices

This patch adds the OF hook to the spi core so that devices
can automatically be registered based on device tree data.  This fixes
a problem with spi devices not binding to drivers after the cleanup of
the spi & i2c binding code.

Signed-off-by: Sinan Akman <sinan@writeme.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>