crossystem: arm: Make FDT_BASE_PATH /proc/device-tree
crossystem: arm: Make FDT_BASE_PATH /proc/device-tree

Hard-coding to /proc/device-tree/firmware/chromeos won't let us read
device-tree properties outside of /firmware/chromeos.

The follow-on CL reads from /firmware/coreboot.

BUG=b:187790074
BRANCH=none
TEST=CQ passes

Change-Id: I752aa5a1e20db1dad7f2508852c7a1c982a4bcb4
Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/4045046
Reviewed-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot