pinctrl: at91: copy define to driver

The #define for the maximum number of GPIO blocks was retrieved
into pinctrl-at91.c by implicit inclusion of <mach/gpio.h>
from <linux/gpio.h> creating a dependency on machine-local
<mach/gpio.h>. Break the depenency by copying this single
define into the driver.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>