pinctrl: canaan: k230: add NULL check in DT parse

pinctrl: canaan: k230: add NULL check in DT parse

Add a NULL check for the return value of of_get_property() when
retrieving the "pinmux" property in the group parser. This avoids
a potential NULL pointer dereference if the property is missing
from the device tree node.

Also fix a typo ("sintenel") in the device ID match table comment,
correcting it to "sentinel".

Fixes: 545887eab6f6 ("pinctrl: canaan: Add support for k230 SoC")
Reported-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Ze Huang <huangze@whut.edu.cn>
Link: https://lore.kernel.org/20250624-k230-return-check-v1-1-6b4fc5ba0c41@whut.edu.cn
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>