pinctrl: sh-pfc: r8a7790: Add missing TX_ER pin to avb_mii group

The pin controller drivers for all R-Car Gen2 SoCs have entries for the
EtherAVB TX_ER pins in their EtherAVB MII groups, except on R-Car H2.

Add the missing pin to restore consistency.

Note that technically TX_ER is an optional signal in the MII bus, and
thus could have its own group, but this is currently not supported by
any R-Car Gen2 pin controller driver.

Fixes: 19ef697d ("sh-pfc: r8a7790: add EtherAVB pin groups")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>