pinctrl: sh-pfc: emev2: Add missing pinmux functions

The err_rst_reqb, ext_clki, lowpwr, and ref_clko pin groups are present,
but no pinmux functions refer to them, hence they can not be selected.

Fixes: 1e7d5d84 ("sh-pfc: Add emev2 pinmux support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>