pcmcia: limit pxa2xx_balloon3 subdriver to balloon3 platform

pxa2xx_balloon3 tries to register pxa2xx-pcmcia device not checking whether
machine is really balloon3, thus messing multi-machine kernels. Fix it up.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>