ram: rockchip: Kconfig: Add RK3399 LPDDR4 entry

Supporting LPDDR4 code support in RK3399 would increases
the size of SPL/TPL.

So add kconfig entry for RK3399 LPDDR4 code so-that
the boards have LPDDR4 can enable them via defconfig.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>
diff --git a/drivers/ram/rockchip/Kconfig b/drivers/ram/rockchip/Kconfig
index 151ffb6..4f274e0 100644
--- a/drivers/ram/rockchip/Kconfig
+++ b/drivers/ram/rockchip/Kconfig
@@ -23,4 +23,11 @@
 	  This enables ram drivers support for the platforms based on
 	  Rockchip RK3399 SoC.
 
+config RAM_RK3399_LPDDR4
+	bool "LPDDR4 support for Rockchip RK3399"
+	depends on RAM_RK3399
+	help
+	  This enables LPDDR4 sdram code support for the platforms based
+	  on Rockchip RK3399 SoC.
+
 endif # RAM_ROCKCHIP