soc: qcom: mem-offline: fix no offlinable region check

When there's only one offlinable region configured, the start and end
section numbers would point to the same section, hence breaking the no
offlinable region check leading to this section not being onlined.
Use the dram physical addresses instead of relying on section numbers.

Example:

On 6GB target with 512MB offlinable region configured:

block_size = 0x20000000, memblock_end_of_DRAM = 0x1e0000000,
bootloader_memory_limit = 0x200000000, memblock_end_pfn = 0x1e0000,
ram_end_pfn = 0x1fffff, start_section_nr = 0xf, end_section_nr = 0xf.

Change-Id: I927d9ad831a913c535f2360fc18587c31c7c2f34
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>