net: lan969x: add RGMII implementation

net: lan969x: add RGMII implementation

The lan969x switch device includes two RGMII port interfaces (port 28
and 29) supporting data speeds of 1 Gbps, 100 Mbps and 10 Mbps. MAC
level delays are configurable through the HSIO_WRAP target, by choosing
a phase shift selector, corresponding to a certain time delay in nano
seconds.

Add new file: lan969x_rgmii.c that contains the implementation for
configuring the RGMII port devices. MAC level delays are configured
using the "{rx,tx}-internal-delay-ps" properties. These properties must
be specified independently of the phy-mode. If missing, or set to zero,
the MAC will not apply any delay.

Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Tested-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20241220-sparx5-lan969x-switch-driver-4-v5-8-fa8ba5dff732@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>