arm64: Override memory limit set by boot parameter

To guarantee successful hot-plugging of memory blocks, these blocks
should belong to ZONE_MOVABLE which has migratable pages. Linux kernel
during boot initializes all the available memory blocks to ZONE_DMA and
ZONE_NORMAL. Hence, boot the system with less memory whose percentage
is given by mem-offline DT property 'mem-percent' and override any memory
limit imposed by kernel boot params and let mem-offline driver initialize
the remaining with ZONE_MOVABLE. The memory blocks in this zone are
applicable for memory offlining.

Change-Id: Ia7b804522fe9425101ee83d1ee354ed09483df13
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>