loads: Block writes into LMB reserved areas of U-Boot

The loads srec loading may overwrite piece of U-Boot accidentally.
Prevent that by using LMB to detect whether upcoming write would
overwrite piece of reserved U-Boot code, and if that is the case,
abort the srec loading.

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