pinctrl: at91: use own header

Copy the mach/at91_pio.h header locally and use it for pinctrl-at91.c. This
allows to remove the dependency on mach/at91_pio.h to be able to move at91 to
multiplatform.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>