Docs: Update user guide for Dynamic Config on FVP

From TF-A v1.5, FVP supports loading the kernel FDT through
firmware as part of dynamic configuration feature. This means
that the FDT no longer needs to be loaded via Model parameters.
This patch updates the user guide to reflect the same.

Change-Id: I79833beeaae44a1564f6512c3a473625e5959f65
Signed-off-by: Soby Mathew <soby.mathew@arm.com>