pinctrl: rzg2l: Support 2.5V PVDD for Ethernet interfaces

pinctrl: rzg2l: Support 2.5V PVDD for Ethernet interfaces

The Ethenet interfaces on the Renesas RZ/G2L SoC family can operate at
multiple power supply voltages: 3.3V (default value), 2.5V and 1.8V.

rzg2l_pinconf_set() is extended to support the 2.5V setting, with a
check to ensure this is only used on Ethernet interfaces as it is not
supported on the SD & QSPI interfaces.

While we're modifying rzg2l_pinconf_set(), drop the unnecessary default
value for pwr_reg as it is set in every branch of the following if
condition.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>