net: rswitch: Add Renesas Ethernet Switch

This patch adds Ethernet Switch support that found on R-Car S4
(r8a779f0) SoC. This is extracted from multiple patches from
downstream BSP, with additional rework of the network device
registration.

Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Phong Hoang <phong.hoang.wz@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[Marek: Rework the driver to support all ports via subdrivers.
        Split the driver up, add generic PHY framework support.
	Generic code clean ups.]
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>