net: phy: mdio-bcm-unimac: Allow configuring MDIO clock divider

[ Upstream commit b78ac6ecd1b6b46f8767cbafa95a7b0b51b87ad8 ]

Allow the configuration of the MDIO clock divider when the Device Tree
contains 'clock-frequency' property (similar to I2C and SPI buses).
Because the hardware may have lost its state during suspend/resume,
re-apply the MDIO clock divider upon resumption.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>