[ARM] pxa: avoid registering multiple pxa2xx_pcmcia devices

cm_x270 and mainstone both register their PCMCIA devices using the same
name, resulting in a warning message from the kernel.  Avoid this by
making the cm_x270 and mainstone PCMCIA initialisation conditional on
the machine type we're running on.

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