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

Add a DTS file for the Spider Ether TSN sub-board (RTP8A779F0ASKB0ST0S),
and include it from the main r8a779f0-spider.dts.

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

Extracted from a larger patch in the BSP by LUU HOAI.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/5aa58816182b34d9e5795bc1e22784f4e4879d13.1643898884.git.geert+renesas@glider.be