net: dpaa2-mac: add backplane link mode support

Add support for backplane link mode, which is, according to discussions
with NXP earlier in the year, is a mode where the OS (Linux) is able to
manage the PCS and Serdes itself.

This commit prepares the ground work for allowing 1G fiber connections
to be used with DPAA2 on the SolidRun CEX7 platforms.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>