imx8m: update clock root and fix core_sel

Update clock root table to let it be easy to configure clock at
very early stage. Also the core_sel mux parent should be A53 CLK
root and ARM PLL.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/include/asm/arch-imx8m/clock_imx8mm.h b/arch/arm/include/asm/arch-imx8m/clock_imx8mm.h
index debed6b..84ed61d 100644
--- a/arch/arm/include/asm/arch-imx8m/clock_imx8mm.h
+++ b/arch/arm/include/asm/arch-imx8m/clock_imx8mm.h
@@ -363,7 +363,8 @@
 	EXT_CLK_2,
 	EXT_CLK_3,
 	EXT_CLK_4,
-	OSC_HDMI_CLK
+	OSC_HDMI_CLK,
+	ARM_A53_ALT_CLK,
 };
 
 enum clk_ccgr_index {