arm64: Add fallback option during memory hot add

Section mapping is good for TLB. But sometimes, when
system is low on physically continuous memory, this can
cause memory allocation failures. Add a fallback option to
allocate vmemmap pages from discontinuous pages.

Change-Id: Ide921c36c1e8980a8af047e2c991310405804858
Signed-off-by: Arun KS <arunks@codeaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>