pinctrl: sh-pfc: r8a7740: Add missing REF125CK pin to gether_gmii group

The gether_gmii_mux[] array contains the REF125CK pin mark, but the
gether_gmii_pins[] array lacks the corresponding pin number.

Fixes: bae11d30 ("sh-pfc: r8a7740: Add GETHER pin groups and functions")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>