net: phy: c45: add support for 1000BASE-T1 forced setup

Add support to force 1000BASE-T1 by setting the correct control bit in
the MDIO_MMD_PMA_PMD_BT1_CTRL register.

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>