Current barebox,environment node only allows specifying a raw device or
partition to load an environment from.  Some boards, like OMAP and
SoCFPGA, instead want to use a file located in a FAT filesystem.
Extend the device tree bindings with a new property 'file-path' that
will trigger this behavior.

This allows any board using this driver to get the env from a file or
from a raw device, instead of each machine type being either raw
device only or file only.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>