SPI driver runtime footprint shrinkage

Shrink the runtime footprint of various SPI drivers:

  - Move the probe() routine into the init section where practical,
    using platform_driver_probe() to make that safe.  This often saves
    around 1KB.  Using platform_driver_probe() can also be a correctness
    fix, if the probe routine is already marked __init but the driver
    struct keeps a dangling pointer to it after init section removal.

  - Likewise move remove() routines into the exit sections.

These changes would be inappropriate iff the platform devices were
actually hotpluggable (e.g. they're found on optional addon cards,
or in an FPGA that's dynamically reprogrammed).  In these cases,
that's not the situation; it's an SOC controller and the only device
is initialized before these drivers.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>