soc: qcom: mem-buf: Align allocation sizes to MHP subsection size

The allocation code hot-adds the memory from another VM through
the memory hotplug code to create the S1 CPU MMU mappings. The memory
hotplug code can only add memory at a subsection granule, meaning
that all memory requests must be subsection size aligned. Thus,
enforce all allocation requests to be subsection size aligned.

Change-Id: Ia4d3294e36265f0bd956fa42670f2ac8d6974908
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>