loader: fix stack management on el1_mmu_enable

There was a missing ldp instruction poping the x27 and x28 values from
the stack.

Signed-off-by: IkerGalardi <contacto.ikergalardi@gmail.com>

Author: IkerGalardi

loader/src/aarch64/util64.S

@@ -371,6 +371,7 @@ BEGIN_FUNC(el1_mmu_enable)
 
     enable_mmu sctlr_el1, x8
 
+    ldp     x27, x28, [sp], #16
     ldp     x29, x30, [sp], #16
     ret