ARM: mmp: delete the custom GPIO header

The MMP sub-architecture appears to have a custom <mach/gpio.h>
for no reason whatsoever. The file became completely empty
after Haojian removed the remaining content in
commit 157d2644
"ARM: pxa: change gpio to platform device".

Cc: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>