ravb: Add MII support for R-Car V4M

ravb: Add MII support for R-Car V4M

All EtherAVB instances on R-Car Gen3/Gen4 SoCs support the RGMII
interface.  In addition, the first two EtherAVB instances on R-Car V4M
also support the MII interface, but this is not yet supported by the
driver.

Add support for MII on R-Car Gen4 by adding an R-Car Gen4-specific EMAC
initialization function that selects the MII clock instead of the RGMII
clock when the PHY interface is MII.  Note that all implementations of
EtherAVB on R-Car Gen4 SoCs have the APSR register, but only MII-capable
instances are documented to have the MIISELECT bit, which has a
documented value of zero when reserved.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://patch.msgid.link/3a21d1d6680864aa85afff9260234c2b8054020a.1719234830.git.geert+renesas@glider.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>