cpu/x86/mp_init: Add comment to smm_do_relocation

It took me a while to understand the SMM set up flow. This adds a
clarifying comment.

BUG=b:194391185, b:179699789
TEST=None

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I9c73e416b8c583cf870e7a29b0bd7dcc99c2f5f4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57858
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

src/cpu/x86/mp_init.c

@@ -714,6 +714,11 @@ static void smm_enable(void)
 		mp_state.do_smm = 1;
 }
 
+/*
+ * This code is built as part of ramstage, but it actually runs in SMM. This
+ * means that ENV_SMM is 0, but we are actually executing in the environment
+ * setup by the smm_stub.
+ */
 static void asmlinkage smm_do_relocation(void *arg)
 {
 	const struct smm_module_params *p;