net: phy: motorcomm: Add pad drive strength cfg support

The motorcomm phy (YT8531) supports the ability to adjust the drive
strength of the rx_clk/rx_data, and the default strength may not be
suitable for all boards. So add configurable options to better match
the boards.(e.g. StarFive VisionFive 2)

When we configure the drive strength, we need to read the current
LDO voltage value to ensure that it is a legal value at that LDO
voltage.

Reviewed-by: Hal Feng <hal.feng@starfivetech.com>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>