Add FDT perl module

There are various models (RTSM VE, FVP Base, Foundation) which the
booterapper is intended to function on, but differences between these
(be they static or configurable) are difficult to handle as the
bootwrapper currently has hard-coded base addresses.

We already have a platform description (the device tree blob) which must
contain the correct addresses for a real OS to function, and it would be
nice if we could extract the values out of the device tree rather than
redundantly describing them in the boot wrapper.

This patch adds a pure perl library for parsing and querying values from
a device tree blob. Scripts may use this library to extract addresses
and other values from the DT.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>