StandaloneMmPkg: Include libfdt in the StMM

The StMM SP will extract boot information from its manifest instead of a C data
structure populated by the SPM. The manifest will be passed by the SPM. This
patch includes support for libfdt to prepare for parsing the manifest in future
patches.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I3b2f57a54569b09661620eccc0d1210a5305026e