[msd-intel-gen] Lock address space operations.

Soon mappings will be created and destroyed potentially on
different threads.

The public methods Alloc, etc. take the lock then delegate to
the internal methods AllocLocked, etc.

MA-465 #comment

Test:
pixelbook: go/magma-tps#L1

Change-Id: Ib45fb955768d21f0e465e8490a832dd5e30ea4ff
8 files changed