spi/pl022: move probe call to subsys_initcall()

The PL022 SPI bus is sometimes used for early stuff like
regulators that need to be present at module_init() time, so
we move this to a subsys_initcall().

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>