fdt: Enhance flashmap function to deal with region properties

Flash regions can optionally be compressed or hashed. Add the ability to
read this information from the flashmap.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>