arm64: dts: renesas: falcon: Add Ethernet sub-board

Add a DTS file for the Falcon Ethernet sub-board (RTP0RC779A0ETS0010S),
and include it from the main r8a779a0-falcon.dts.

For now its contents are limited to the Board ID EEPROM.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210304153257.4059277-4-geert+renesas@glider.be