mb/google/auron: Convert variants to use override devicetree

mb/google/auron: Convert variants to use override devicetree

Since the variants' devicetrees are almost identical, convert to
using an overridetree setup for simplicity.

As part of the cleanup, drop unused PCIe RP5 for buddy as well.

Test: build all auron variants, compare generated static.c to ensure
resulting generated contents unchanged (although layout will)

Change-Id: I290e7243335a64afdcfc629db7b8ce18f5aa993c
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39940
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>