drivers: ddr: introduce DDR driver for i.MX8M

Introduce DDR driver for i.MX8M. The driver will be used by SPL to
initialze DDR PHY and DDR Controller.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/drivers/ddr/imx/Kconfig b/drivers/ddr/imx/Kconfig
new file mode 100644
index 0000000..7e06fb2
--- /dev/null
+++ b/drivers/ddr/imx/Kconfig
@@ -0,0 +1 @@
+source "drivers/ddr/imx/imx8m/Kconfig"