pinctrl: at91: allow to have disabled gpio bank

Today we expect that all the bank are enabled, and count the number of banks
used by the pinctrl based on it instead of using the last bank id enabled.

So switch to it, set the chained IRQ at runtime based on enabled banks
and wait only the number of enabled gpio controllers at probe time.

Cc: <stable@vger.kernel.org> # 3.18
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>