ARM: mmp: remove mach/gpio-pxa.h

Commit 157d2644 ("ARM: pxa: change gpio
to platform device") removed all includes of mach/gpio-pxa.h. It kept
this unused header in the tree. Using it can't work, as it itself
includes the non-existent header plat/gpio-pxa.h. This header can safely
be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>