board: visionfive2: configure PHY pad drive strength

Configure the pad drive strength register for both PHYs.
The values correspond to what can be found in the Linux DTS
for VisionFive2 v1.3b.

Pad drive strength configuration is required for the phy0 to work correctly
with 100Mbit links.

Signed-off-by: Lukasz Tekieli <tekieli.lukasz@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>