xen/arm32: head: Add mising isb in switch_to_runtime_mapping()

Per the Arm Arm (ARM DDI 0406C.d A3.8.3):

"The DMB and DSB memory barriers affect reads and writes to the memory
system generated by load/store instructions and data or unified cache
maintenance operations being executed by the processor. Instruction
fetches or accesses caused by a hardware translation table access are
not explicit accesses."

The function switch_to_runtime_mapping() is responsible to map the
Xen at its runtime address if we were using the temporary area before
jumping returning using a runtime address. So we need to ensure the
'dsb' has completed before continuing. Therefore add an 'isb'.

Fixes: fbd9b5fb4c ("xen/arm32: head: Remove restriction where to load Xen")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/arm32/head.S

@@ -696,6 +696,11 @@ switch_to_runtime_mapping:
 
         /* Ensure any page table updates are visible before continuing */
         dsb   nsh
+        /*
+         * The function will return on the runtime mapping. So we want
+         * to prevent instruction fetch before the dsb completes.
+         */
+        isb
 
 ready_to_switch:
         mov   pc, lr