x86: fsp: Add comments about U-Boot entering start.S twice

Add some comments in start.S for the fact that with FSP U-Boot
actually enters the code twice. Also change to use fsp_init()
and fsp_continue for accuracy.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/start.S b/arch/x86/cpu/start.S
index e2b5ef4..d072825 100644
--- a/arch/x86/cpu/start.S
+++ b/arch/x86/cpu/start.S
@@ -115,8 +115,10 @@
 #endif
 #else
 	/*
-	 * When we get here after car_init(), esp points to a temporary stack
-	 * and esi holds the HOB list address returned by the FSP.
+	 * U-Boot enters here twice. For the first time it comes from
+	 * car_init_done() with esp points to a temporary stack and esi
+	 * set to zero. For the second time it comes from fsp_init_done()
+	 * with esi holding the HOB list address returned by the FSP.
 	 */
 #endif
 	/* Set up global data */