arm64: Fix page boundary check when hot onlining

During page online callback, the physical address of the
page is checked if it is within the RAM end boundary set by
the bootloader. The current check is broken as it misses
to online the last PFN, thereby making this page being reserved
all the time. Fix this broken check.

Change-Id: I36ed5ae3f3ed0f199dacd394358690761ddcd80c
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>