lmb: Reserve U-Boot separately if relocation is disabled

In case U-Boot starts with GD_FLG_SKIP_RELOC, the U-Boot code is
not relocated, however the stack and heap is at the end of DRAM
after relocation. Reserve a LMB area for the non-relocated U-Boot
code so it won't be overwritten.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>