fdt: parse "reserved-memory" for memory reservation

boot_fdt_add_mem_rsv_regions() adds reserved memory sections to an lmb
struct. Currently, it only parses regions described by /memreserve/
entries.

Extend this to the more commonly used scheme of the "reserved-memory"
node.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>