Fix soc_common PCMCIA configuration

Jonathan Cameron reports that building PCMCIA as modules doesn't work:

As module get a load of undefined symbols:
ERROR: "soc_pcmcia_request_irqs" [drivers/pcmcia/pxa2xx_stargate2.ko] undefined!
ERROR: "soc_pcmcia_free_irqs" [drivers/pcmcia/pxa2xx_stargate2.ko] undefined!
ERROR: "soc_pcmcia_enable_irqs" [drivers/pcmcia/pxa2xx_stargate2.ko] undefined!
ERROR: "soc_pcmcia_disable_irqs" [drivers/pcmcia/pxa2xx_stargate2.ko] undefined!
ERROR: "soc_pcmcia_add_one" [drivers/pcmcia/pxa2xx_base.ko] undefined!
ERROR: "soc_common_pcmcia_get_timing" [drivers/pcmcia/pxa2xx_base.ko] undefined!
ERROR: "soc_pcmcia_remove_one" [drivers/pcmcia/pxa2xx_base.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2

This is because soc_common tries to be built-in, but it should be a module.
Allow soc_common to be a module.

Reported-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>