file: add new helper functions to grok FDT nodes

This file is intended to help find SPI devices using the FDT.
Here's how it works:

Under /proc/device-tree there is a node called "aliases" where
each file is a device name with an instance (e.g. "spi0"). The
content of the file is the controller's memory-mapped address.

That info can be used to find the appropriate node directory to
search under /proc/device-tree, where we can recursively search
for "compatible" nodes and scan for the matching user-provided
compatible string.

For SPI devices, we need to look for the matching "compatible"
node and then read the corresponding "reg" node to obtain chip-
select number.

BUG=chrome-os-partner:43453
BRANCH=none
TEST=reads now work on danger without specifying a SPI device
Signed-off-by: David Hendricks <dhendrix@chromium.org>

Change-Id: I1f771bcca61de71daa54e358e058723d3fd37079
Reviewed-on: https://chromium-review.googlesource.com/310072
Commit-Ready: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
---
M file.c
M file.h
2 files changed, 160 insertions(+), 2 deletions(-)