spi_mpc83xx: handle other Freescale processors

With this patch we'll able to select spi_mpc83xx driver on the MPC86xx
platforms.  Let the driver depend on FSL_SOC, so we don't have to worry
about Kconfig anymore.

Also remove the "experimental" dependency, the driver has been tested to
work on a various hardware, and surely not experimental anymore.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>