lmb: handle more than one DRAM BANK

This fixes the automatic lmb initialization and reservation for boards
with more than one DRAM bank.

This fixes the CVE-2018-18439 and -18440 fixes that only allowed to load
files into the firs DRAM bank from fs and via tftp.

Found-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>