arm: clarify comment about exception stack use

Each mode has its own stack, but we only use it as a base pointer
to a private memory region. That region, which has a size of
sizeof(struct pt_regs), is used to store some registers during the
switch to svc mode - which will use the normal svc stack. The switch
to svc mode is done because we handle all exceptions in svc.
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arm/cstart.S

@@ -121,8 +121,9 @@ asm_mmu_enable:
  * Simplified version of the Linux kernel implementation
  *   arch/arm/kernel/entry-armv.S
  *
- * Each mode has an S_FRAME_SIZE sized stack initialized
- * in exceptions_init
+ * Each mode has an S_FRAME_SIZE sized memory region,
+ * and the mode's stack pointer has been initialized
+ * to the base of that region in exceptions_init.
  */
 .macro vector_stub, name, vec, mode, correction=0
 .align 5