spi: core and gpio expanders use subsys_init

Make the SPI external GPIO expander drivers register themselves at
subsys_initcall() time when they're statically linked, and make the SPI
core do its driver model initialization earlier so that's safe.

SOC-integrated GPIOs are available starting very early -- often before
initcalls start to run, or earily in arch_initcall() at latest -- so this
improves consistency, letting more subsystems rely on GPIOs being usable
by their own subsys_initcall() code.

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>