mm/Kconfig: support forcing allocators to return ZONE_DMA memory

Add a new config item, CONFIG_FORCE_ALLOC_FROM_DMA_ZONE, which
can be used to optionally force certain allocators to always
return memory from ZONE_DMA.

This option helps ensure that clients who require ZONE_DMA
memory are always using ZONE_DMA memory.

Change-Id: Id2d36214307789f27aa775c2bef2dab5047c4ff0
Signed-off-by: Praveen Kurapati <pkurapat@codeaurora.org>
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>