rtc: omap: use devm_pinctrl_register()

Use a managed variant of pinctrl_register(). This way we can shorten
the remove() callback as well as drop a goto label from probe().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201109163409.24301-2-brgl@bgdev.pl