iommu/arm-smmu: Don't wipe out MICRO_MMU_CTRL while halting

The MICRO_MMU_CTRL register is used for halt control, but it also
contains some important configuration bits.  Currently, when we request
a halt, we're wiping out all of the other bits in the register, losing
any previous configuration.  Fix this by reading the value and OR'ing in
the bits we need.

Change-Id: Iafdc5b2a344916dce9bac7f958a153dbee042ca8
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>