libfdt: Add FDT_CREATE_FLAG_NO_NAME_DEDUP flag that trades size for speed

Searching for duplicate names scales O(n^2) with the number of names
added to a fdt, which can cause a noticable slowdown with larger device
trees and very slow CPU cores.

Add FDT_CREATE_FLAG_NO_NAME_DEDUP that allow the caller to trade fdt size
for speed in the creation process.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20190509094122.834-4-npiggin@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>