Upcoming patches will put I2C/SPI on their own busses with
spi_register_driver / i2c_register_driver which will only
be available if the subsystem is enabled. We could provide
static inlines, but it wouldn't make much sense to compile
a spi/i2c driver if the corresponding subsystem is disabled.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>