rpmsg-kdrv: add support for remote eth device

In J721E, the remote R5F can run firmware which controls 9-port CPSW9G NUSS
module - Eth switch fw. Only the Eth Switch FW has full control over J721E
CPSW9G and allows segregate Ethernet traffic for each attached Remote CPU
by using programable ALE classifiers and exclusive UDMA TX channels and RX
flows. The Eth switch FW supports rpmsg-kdrv protocol and presents itself
as rpmsg-kdrv device, which provides CPSW9g resource management and
debugging functions for each attached Remote CPU:
- DMA resources allocation: TX UDMA channel and RX Flow
- MAC addresses assignment
- ARP helper for ARP requests processing from remote Net Hosts
- dbg: IPC ping
- dbg: CPSW9G register access
- dbg: statistic print request on R5F console (ioctl)

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>