dt: amd-seattle: remove Overdrive revision A0 support

Support for AMD Seattle silicon revision A0 is no longer relevant,
since we no longer have a driver for the network controller, and
the PCIe on these boards was very unreliable. So drop the DTS
description of the A0 version of the overdrive board.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>