net: ethernet: rtsn: Add support for Renesas Ethernet-TSN

Add initial support for Renesas Ethernet-TSN End-station device of R-Car
V4H. The Ethernet End-station can connect to an Ethernet network using a
10 Mbps, 100 Mbps, or 1 Gbps full-duplex link via MII/GMII/RMII/RGMII.
Depending on the connected PHY.

The driver supports Rx checksum and offload and hardware timestamps.

While full power management and suspend/resume is not yet supported the
driver enables runtime PM in order to enable the module clock. While
explicit clock management using clk_enable() would suffice for the
supported SoC, the module could be reused on SoCs where the module is
part of a power domain.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>