NOTE: Breaking change to the way UUIDs are stored in the DT

Currently, UUIDs are stored in the device tree as
sequences of 4 integers. There is a mismatch in endianness
between the way UUIDs are represented in memory and the way
they are parsed from the device tree. As a result, we must either
store the UUIDs in little-endian format in the DT (which means
that they do not match up with their string representations)
or perform endianness conversion after parsing them.

Currently, TF-A chooses the second option, with unwieldy
endianness-conversion taking place after reading a UUID.

To fix this problem, and to make it convenient to copy and
paste UUIDs from other tools, change to store UUIDs in string
format, using a new wrapper function to parse them from the
device tree.

Change-Id: I38bd63c907be14e412f03ef0aab9dcabfba0eaa0
Signed-off-by: David Horstmann <david.horstmann@arm.com>