Add fdt_add_reserved_memory() helper function

If a firmware component like TF-A reserves special memory regions for
its own or secure payload services, it should announce the location and
size of those regions to the non-secure world. This will avoid
disappointment when some rich OS tries to acccess this memory, which
will likely end in a crash.

The traditional way of advertising reserved memory using device tree is
using the special memreserve feature of the device tree blob (DTB).
However by definition those regions mentioned there do not prevent the
rich OS to map this memory, which may lead to speculative accesses to
this memory and hence spurious bus errors.

A safer way of carving out memory is to use the /reserved-memory node as
part of the normal DT structure. Besides being easier to setup, this
also defines an explicit "no-map" property to signify the secure-only
nature of certain memory regions, which avoids the rich OS to
accidentally step on it.

Add a helper function to allow platform ports to easily add a region.

Change-Id: I2b92676cf48fd3bdacda05b5c6b1c7952ebed68c
Signed-off-by: Andre Przywara <andre.przywara@arm.com>