Move barriers.h to aarch64 inc folder

The file now contains aarch64 assembly and should not be in the
architecture-agnostic folder.

Change-Id: I4c5c7d98ffe75d95ee35fc360c025a8c68c10cb3
diff --git a/inc/hf/arch/barriers.h b/src/arch/aarch64/inc/hf/arch/barriers.h
similarity index 100%
rename from inc/hf/arch/barriers.h
rename to src/arch/aarch64/inc/hf/arch/barriers.h