of/spi: Fix SPI module loading by using proper "spi:" modalias prefixes.

To get modprobe to automatically load the proper modules, we need to
prefix things with "spi:".  Partially based on Grant Likely's suggestions.

Signed-off-by: David Daney <david.daney@cavium.com>
[grant.likely: reworked because drivers/of/of_spi.c has been removed]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>