TF-A does not have the capability to read UUIDs in string form
from the device tree. This capability is useful for readability,
so add a wrapper function, fdtw_read_uuid() to parse UUIDs from
the DT.
This function should parse a string of the form:

"aabbccdd-eeff-4099-8877-665544332211"

to the byte sequence in memory:

[aa bb cc dd ee ff 40 99 88 77 66 55 44 33 22 11]

Change-Id: I99a92fbeb40f4f4713f3458b36cb3863354d2bdf
Signed-off-by: David Horstmann <david.horstmann@arm.com>