lmb: add and reserve memory above ram_top

U-Boot does not use memory above ram_top. However, this memory does
need to get registered as part of the memory map, so that subsystems
like EFI pass it on to the operating system as part of the EFI memory
map. Add memory above ram_top and reserve it with the LMB_NOOVERWRITE
flag so that it does not get allocated or re-used.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Suggested-by: Mark Kettenis <kettenis@openbsd.org>