pinctrl: at91: Use devm_pinctrl_register() for pinctrl registration

Use devm_pinctrl_register() for pin control registration and remove
the need of .remove callback.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>