arm64: Honor limits set by bootloader

Introduce a variable to save bootloader enforced memory limits and
restricts adding beyond this boundary during a memory hotplug. Also,
export this symbol so that other kernel module have access to it.

Change-Id: I28c100644b7287ec4625c4c018b5fffc865e2e72
Signed-off-by: Arun KS <arunks@codeaurora.org>
[sudaraja@codeaurora.org: check limit with physical address of page]
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
[swatsrid@codeaurora.org: Fix merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>