soc: qcom: mem-offline: Update end pfn calculation

On a 6GB target with 0x80000000 as the base address, the
dram end address becomes 0x200000000. Now doing a phys_to_pfn
on this address gives a PFN which sits on this end address, which
is not a valid one. Hence, update the calculation of getting
the end PFN of the system.

Change-Id: Iada40892bf12dd450193b2ae3741c59c6295f604
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>