Fixing various warnings

* Missing and incorrect # Safety comments
* Unused functions and variables
* Refactoring conditional AArch64 compilation

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ie638d1eeb2928a0c6eb0d4eb64c9d19b82f91e4d
diff --git a/src/kernel_space.rs b/src/kernel_space.rs
index 34433ff..f9e1d68 100644
--- a/src/kernel_space.rs
+++ b/src/kernel_space.rs
@@ -121,6 +121,7 @@
     /// This changes the mapping of the running execution context. The caller
     /// must ensure that existing references will be mapped to the same address
     /// after activation.
+    #[cfg(target_arch = "aarch64")]
     pub unsafe fn activate(&self) {
         self.xlat.lock().activate();
     }