net: mdio: mscc-miim: add lan966x internal phy reset support

The LAN966x has two internal PHYs which are in reset by default. The
driver already supported the internal PHYs of the SparX-5. Now add
support for the LAN966x, too. Add a new compatible to distinguish them.

The LAN966x has additional control bits in this register, thus convert
the regmap_write() to regmap_update_bits() to leave the remaining bits
untouched. This doesn't change anything for the SparX-5 SoC, because
there, the register consists only of reset bits.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>