phy: add support for backplane kr mode

Add generic support for backplane kr modes currently available:
10gbase-kr, 40gbase-kr4. Remove platform generic fixups
(armv8/layerscape and powerpc) for ethernet interfaces specified
in device tree as supported backplane modes.

Signed-off-by: Florinel Iordache <florinel.iordache@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>