clk: mediatek: add CLK_XTAL support for clock driver

This adds the CLK_XTAL macro/flag to allow modeling clocks which are
directly connected to the xtal clock.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
diff --git a/drivers/clk/mediatek/clk-mtk.h b/drivers/clk/mediatek/clk-mtk.h
index e7c61ae..48ce164 100644
--- a/drivers/clk/mediatek/clk-mtk.h
+++ b/drivers/clk/mediatek/clk-mtk.h
@@ -29,7 +29,8 @@
 #define CLK_PARENT_APMIXED		BIT(4)
 #define CLK_PARENT_TOPCKGEN		BIT(5)
 #define CLK_PARENT_INFRASYS		BIT(6)
-#define CLK_PARENT_MASK			GENMASK(6, 4)
+#define CLK_PARENT_XTAL			BIT(7)
+#define CLK_PARENT_MASK			GENMASK(7, 4)
 
 #define ETHSYS_HIFSYS_RST_CTRL_OFS	0x34