pinctrl: rza1: Handle devm_kasprintf() failure cases

devm_kasprintf() may return NULL on failure of internal allocation
thus the assignments are not safe if not checked. On error
rza1_pinctrl_register() respectively rza1_parse_gpiochip() return
negative values so -ENOMEM in the (unlikely) failure case of
devm_kasprintf() should be fine here.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 5a49b644 ("pinctrl: Renesas RZ/A1 pin and gpio controller")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>