powerpc/85xx: fsl_corenet_serdes code rework

Rework and add some new APIs to the fsl_corenet_serdes code for use by
erratum and drivers.

* Rename serdes_get_bank() to serdes_get_bank_by_lane()
* Add serdes_get_first_lane returns which SERDES lane is used by device

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>