[ARM] pxa: restrict availability of pxa2xx PCMCIA drivers

Not all PXA2xx platforms provide the necessary support files for
PCMCIA to work, let alone build.  Stop the build errors by making
PXA2xx PCMCIA support only available on those platforms which have
supplied the support files.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>