phy: Add RGMII support on lan966x

The serdes driver contains also a mux to decide which interface type to
use. Currently the driver supports GMII/SGMII/QSGMII and partially RGMII.
As it doesn't support all the other RGMII interfaces like
RGMII_TXID/RXID/ID and it could run only at 1G.
Therefore extend this for all the other speeds(10/100) and also allow
the other interfaces.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20220901121455.245103-1-horatiu.vultur@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
