ls1046aqds: add support for backplane kr

Add support for backplane kr on ls1046aqds: remove board specific fixups
on ls1046aqds 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>