imx8ulp_evk: Change to use DDR driver

Remove the DDR initialization codes from board and enable the iMX8ULP
DDR driver.

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/arch/arm/mach-imx/imx8ulp/Kconfig b/arch/arm/mach-imx/imx8ulp/Kconfig
index bbdeaac..c1c1aa0 100644
--- a/arch/arm/mach-imx/imx8ulp/Kconfig
+++ b/arch/arm/mach-imx/imx8ulp/Kconfig
@@ -20,6 +20,7 @@
 	bool "imx8ulp_evk"
 	select IMX8ULP
 	select SUPPORT_SPL
+	select IMX8ULP_DRAM
 
 endchoice