ARM: bootm: don't assume sp is in DRAM bank 0

arch_lmb_reserve() currently assumes that the stack pointer is within DRAM
bank 0. This is not necessarily true. Enhance the code to search through
DRAM banks until the bank that does contain SP is found, and then reserve
the tail of that bank.

Fixes: 2d1916e4 ("ARM: add flat device tree support")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>