ddr: imx8m: hide i.MX8M DDR options from device driver entry

Use one menu to hide the several i.MX8M DDR options from device
driver entry.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/drivers/ddr/imx/imx8m/Kconfig b/drivers/ddr/imx/imx8m/Kconfig
index 71f466f..a83b0f4 100644
--- a/drivers/ddr/imx/imx8m/Kconfig
+++ b/drivers/ddr/imx/imx8m/Kconfig
@@ -1,3 +1,6 @@
+menu "i.MX8M DDR controllers"
+	depends on ARCH_IMX8M
+
 config IMX8M_DRAM
 	bool "imx8m dram"
 
@@ -20,3 +23,4 @@
 	  info into memory for low power use. OCRAM_S is used for this
 	  purpose on i.MX8MM.
 	default 0x180000
+endmenu