pinctrl: u300: 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>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>