lmb: prohibit allocations above ram_top even from same bank

There are platforms which set the value of ram_top based on certain
restrictions that the platform might have in accessing memory above
ram_top, even when the memory region is in the same DRAM bank. So,
even though the LMB allocator works as expected, when trying to
allocate memory above ram_top, prohibit this by marking all memory
above ram_top as reserved, even if the said memory region is from the
same bank.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Andreas Schwab <schwab@suse.de>