rockchip: Fix incorrect constant name in RAM init code

A condition in the rk3399 RAM initialization code used the old
CONFIG_RAM_RK3399_LPDDR4 constant name. This commit changes the
condition to use the correct CONFIG_RAM_ROCKCHIP_LPDDR4 constant.

Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/ram/rockchip/sdram_rk3399.c b/drivers/ram/rockchip/sdram_rk3399.c
index b1fea04..963a05c 100644
--- a/drivers/ram/rockchip/sdram_rk3399.c
+++ b/drivers/ram/rockchip/sdram_rk3399.c
@@ -2954,7 +2954,7 @@
 		params->ch[ch].cap_info.rank = rank;
 	}
 
-#if defined(CONFIG_RAM_RK3399_LPDDR4)
+#if defined(CONFIG_RAM_ROCKCHIP_LPDDR4)
 	/* LPDDR4 needs to be trained at 400MHz */
 	lpddr4_set_rate(dram, params, 0);
 	params->base.ddr_freq = dfs_cfgs_lpddr4[0].base.ddr_freq / MHz;