net: ti: prueth_core: Add support for RSTP switch

PRU based ethernet subsystem can operate in RSTP switch using a
different firmware (than Dual EMAC mode). RSTP switch support L2
forwarding offloading and RSTP state maintenance.

This patch adds switch support for PRUETH driver using switchdev
framework in order to support RSTP switch mode.

Driver supports:
- FDB (forwarding database) offload
- RSTP

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
[vigneshr@ti.com:
 - Move to new switchdev APIs
 - Use static RX/TX/Host queue sizes
 - Split switch related code to separate function
 - Rebase to v5.4]
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>