ARM: mvebu: remove unused mach/gpio.h

This file was left over from a cleanup of asm/gpio.h and has
not been used in a while. Let's just remove it now, so the
arch/arm/mach-mvebu/include/ directory can also disappear.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>