device_tree: Add function to get top of memory from a FDT blob

coreboot needs to figure out top of memory to place CBMEM data. On some
non-x86 QEMU virtual machines, this is achieved by probing the RAM space
to find where the VM starts discarding data since it's not backed by
actual RAM. This behaviour seems to have changed on the QEMU side since
then, VMs using the "virt" model have started raising exceptions/errors
instead of silently discarding data (likely [1] for example) which has
broken coreboot on these emulation boards.

The qemu-aarch64 mainboard is intended for the "virt" model and had this
issue, which was fixed by using exception handlers in the RAM detection
process [2].

The qemu-riscv mainboard is also for "virt" and still has this issue.
There is a potential fix based on the exception handler approach [3],
but it fails to build for 32-bit RISC-V. There's also a WIP attempt on
parsing the in-memory device-tree that QEMU provides us [4], but it
relies on unflattening which wouldn't work on romstage.

The qemu-armv7 mainboard code is intended for the "vexpress-a9" model VM
which doesn't appear to suffer from this issue. Still, the issue can be
observed on the ARMv7 "virt" model via a port based on qemu-aarch64.

QEMU docs for ARM and RISC-V "virt" models [5][6] recommend reading the
device tree blob it provides for device information (incl. RAM size).
Implement a function that parses the device tree blob to find the top of
memory in order to use it in mainboard code as an alternative to probing
RAM space. ARM64 code initializes CBMEM in romstage where malloc isn't
available, so take care to do parsing without unflattening the blob and
make the code available in romstage as well.

This assumes a single memory node with a single reg range which seems
to be enough for what QEMU provides for now, support for more complex
device-trees (as supported by specification) is left as future work.

[1] https://lore.kernel.org/qemu-devel/1504626814-23124-1-git-send-email-peter.maydell@linaro.org/T/#u
[2] https://review.coreboot.org/c/coreboot/+/34774
[3] https://review.coreboot.org/c/coreboot/+/36486
[4] https://review.coreboot.org/c/coreboot/+/78981
[5] https://qemu-project.gitlab.io/qemu/system/arm/virt.html
[6] https://qemu-project.gitlab.io/qemu/system/riscv/virt.html

Change-Id: I8bef09bc1bc4e324ebeaa37f78d67d3aa315f52c
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
---
M src/include/device_tree.h
M src/lib/Makefile.mk
M src/lib/device_tree.c
3 files changed, 102 insertions(+), 1 deletion(-)