spi/bfin_spi: init early

Some systems using this bus sometimes have very basic devices on them
such as regulators.  So we need to be loaded even earlier in case the
devices are used by things such as early board init code.  Therefore
register in subsys_initcall().

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>