arm: bootm: fix sp detection at end of address range

This fixes  'arch_lmb_reserve()' for ARM that tries to detect in which
DRAM bank 'sp' is in.

This code failed if a bank was at the end of physical address range
(i.e. size + length overflowed to 0).

To fix this, calculate 'bank_end' as 'size + length - 1' so that such
banks end at 0xffffffff, not 0.

Fixes: 15751403 ("ARM: bootm: don't assume sp is in DRAM bank 0")
Reported-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>