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.

Fixes: 490e687e ("pinctrl: sh-pfc: Initial R-Car M3-N support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>