ARM: at91: fix pinctrl and old gpio simultaneous support

No more compiling the old gpio driver when pinctrl is used leads to too many
compilation issues with current defconfigs. So still compile it but decide at
runtime if which driver to use.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>