libfdt: Downgrade to version 1.4.6-9

Version 1.4.7 introduces a big performance hit to functions that access
the FDT. Downgrade the library to version 1.4.6-9, before the changes
that introduce the problem. Version 1.4.6 isn't used because one of the
libfdt files (fdt_overlay.c) is missing the license header. This
problem is also fixed in 1.4.6-9.

This version corresponds to commit <aadd0b65c987> checks: centralize
printing of property names in failure messages.

Fixes ARM-software/tf-issues#643

Change-Id: I73c05f2b1f994bcdcc4366131ce0647553cdcfb8
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>