SPI: Add helper macro for spi_driver boilerplate

This patch introduces the module_spi_driver macro which is a convenience macro
for SPI driver modules similar to module_platform_driver. It is intended to be
used by drivers which init/exit section does nothing but register/unregister
the SPI driver. By using this macro it is possible to eliminate a few lines of
boilerplate code per SPI driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>