cpu/x86/smm_stub.S: Update comment

%ebp is used for the stack frame on which the fxrstor address is pushed.
entry64.inc does not trash it so that's fine.

Change-Id: If027437dccac9ad507ceb534c6aae77ea43bdfda
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68896
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S
index 19e9c50..1411cbc 100644
--- a/src/cpu/x86/smm/smm_stub.S
+++ b/src/cpu/x86/smm/smm_stub.S
@@ -230,7 +230,7 @@
 	movl	%eax, -0x8(%ebp)
 	movl	%edx, -0xc(%ebp)
 
-	/* entry64.inc preserves ebx, esi, edi */
+	/* entry64.inc preserves ebx, esi, edi, ebp */
 #include <cpu/x86/64bit/entry64.inc>
 	mov	%edi, %ecx