ARM: at91: localize GPIO header

This moves the <mach/gpio.h> header in the AT91 platform down
into the machine directory and removes the reliance on
MACH_NEED_GPIO_H from the AT91.

This does not move the platform to GENERIC_GPIO but localize
the remaining work to be done for this to the mach-at91
folder.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[nicolas.ferre@atmel.com: adapt to newer kernel, add rsi-ews board]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>