efi_loader: Reserve unaccessible memory

On some systems, not all RAM may be usable within U-Boot. Maybe the
memory maps are incomplete, maybe it's used as workaround for broken
DMA. But whatever the reason may be, a platform can say that it does
not wish to have its RAM accessed above a certain address by defining
board_get_usable_ram_top().

In the efi_loader world, we ignored that hint, mostly because very few
boards actually have real restrictions around this.

So let's honor the board's wish to not access high addresses during
boot time. The best way to do so is by indicating the respective pages
as "allocated by firmware". That way, Operating Systems will still
use the pages after boot, but before boot no allocation will use them.

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Baruch Siach <baruch@tkos.co.il>