net: phy: micrel: Configure KSZ9021/KSZ9031 skew from OF

Add code to process the KSZ9021/KSZ9031 OF props if they are present
and configure skew registers based on the information from the OF.
This code is only enabled if the DM support for ethernet is also
enabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>

V2: - Implement struct ksz90x1_reg_field to describe the skew register
      fields more accurately.
    - Fix RXDV/TXEN skew register default value and offset.