net: ravb: Drop SoC-specific compatible support

The current set of U-Boot upstream R-Car Gen3 DTs all contain generic
"renesas,etheravb-rcar-gen3" compatible strings, drop the SoC specific
compatible string support from U-Boot to reduce size and duplication.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>