sgi/install_prerequisites.sh: install libfdt library for gcc toolchain

Add helper function to download, build and install LIBFDT library for
GCC 10.2 toolchain. This library is required for the building of
kvmtool.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Ibfb56b10b22ddc58af3c0e6a28d4cdb280bf374f