net: mvpp2: use new MVMDIO driver

This commit ports mvpp2 to use the recently introduced Marvell MDIO
(MVMDIO) driver.  It removes direct interaction with the SMI & XSMI
busses.  This commit is based in part on earlier work by
Ken Ma <make@marvell.com> in Marvell's own downstream repo:
https://github.com/MarvellEmbeddedProcessors/u-boot-marvell/commit/c81dc39

.

The above refrenced work was based on an MVMDIO implementation that
never made it into U-Boot.  With this patch the mvpp2 driver switches
to use the new MVMDIO driver that is based on a more universal
mdio-uclass implementation.

Signed-off-by: Nevo Hed <nhed+github@starry.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>