arm64: remap strongly ordered memory

When clients request strongly ordered memory, they generally
want it to avoid speculative accesses from the CPU. Speculative
accesess could come from the existing mapping though so make
sure to get rid of the original mapping before creating the
strongly ordered mappping.

Change-Id: I8e219838b53b24da61003fbf2006ab1709761603
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>