[ARM] pxa/zeus: make Viper pcmcia support more generic to support Zeus

The Arcom Zeus CF slot requires the same kind of support as the Viper.
To avoid code duplication, introduce a platform device that abstracts
the differences.

This also allows for the removal of the ugly export of viper_cf_rst().

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>