x86/boot: clarify comment about trampoline_setup usage

Clarify that trampoline_setup is only used for EFI when booted using the
multiboot2 entry point.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/head.S

@@ -504,9 +504,7 @@ trampoline_bios_setup:
         mov     %ecx,sym_esi(trampoline_phys)
 
 trampoline_setup:
-        /*
-         * Called on legacy BIOS and EFI platforms.
-         */
+        /* Called for Mutiboot entry, including MB2+EFI. */
 
         /* Save Xen image load base address for later use. */
         mov     %esi, sym_esi(xen_phys_start)