ARM: pxa: define palmte2_pxa_keys conditionally

Gcc prints a harmless warning about palmte2_pxa_keys not being used
when the gpio keyboard driver is disabled. The solution is to use
the same #ifdef that is already present in the place where the
symbol is used.

Without this patch, building palmz72_defconfig results in:

/home/arnd/linux-arm/arch/arm/mach-pxa/palmte2.c:128:31: warning: 'palmte2_pxa_keys' defined but not used [-Wunused-variable]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Carlos Eduardo Medaglia Dyonisio <cadu@nerdfeliz.com>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Eric Miao <eric.y.miao@gmail.com>