ARM: pxa: mark spitz_card_pwr_ctrl as __maybe_unused

This function is only used when CONFIG_PCMCIA is enabled, otherwise
we get a harmless warning:

arch/arm/mach-pxa/spitz.c:204:13: warning: 'spitz_card_pwr_ctrl' defined but not used [-Wunused-function]

Marking it as __maybe_unused keeps the logic simple and avoids the
warning on randconfig builds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>