pinctrl: sh-pfc: r8a77965: Replace DU_DOTCLKIN2 by DU_DOTCLKIN3
Unlike R-Car M3-W, R-Car M3-N does not have DU_DOTCLKIN2, but the
corresponding pin carries the DU_DOTCLKIN3 signal. Correct all
references to DU_DOTCLKIN2 to fix this.
This change does not have any runtime effect, as it only changes an
internal enum name, and a comment.