ARM: 7043/1: mach-ixp2000: rename GPIO header

The ixp2000 abuses the <mach/gpio.h> namespace by not implementing
any generic GPIO nor gpiolib functions in it - just custom GPIO.
Rename the header to <mach/gpio-ixp2000.h> for clarity.

Cc: Lennert Buytenhek <kernel@wantstofly.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>