sifive: fu540: add ddr driver

Add driver for fu540 to support ddr initialization in SPL.
This driver is based on FSBL
(https://github.com/sifive/freedom-u540-c000-bootloader.git)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/ram/Kconfig b/drivers/ram/Kconfig
index 56fea7c..7e6e981 100644
--- a/drivers/ram/Kconfig
+++ b/drivers/ram/Kconfig
@@ -74,4 +74,5 @@
 	  This driver is for the sdram memory interface with the SEMC.
 
 source "drivers/ram/rockchip/Kconfig"
+source "drivers/ram/sifive/Kconfig"
 source "drivers/ram/stm32mp1/Kconfig"