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

In the Linux kernel, the following vulnerability has been resolved:

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".