xpl: Rename spl_phase() to xpl_phase()

Rename this function to indicate that it refers to any xPL phase.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/apollolake/punit.c b/arch/x86/cpu/apollolake/punit.c
index b1503c2..9c708e9 100644
--- a/arch/x86/cpu/apollolake/punit.c
+++ b/arch/x86/cpu/apollolake/punit.c
@@ -77,7 +77,7 @@
 
 static int apl_punit_probe(struct udevice *dev)
 {
-	if (spl_phase() == PHASE_SPL)
+	if (xpl_phase() == PHASE_SPL)
 		return punit_init(dev);
 
 	return 0;