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/board/sifive/fu540/Kconfig b/board/sifive/fu540/Kconfig
index 4330ac4..d41c305 100644
--- a/board/sifive/fu540/Kconfig
+++ b/board/sifive/fu540/Kconfig
@@ -19,6 +19,8 @@
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
 	select GENERIC_RISCV
+	select RAM
+	select SPL_RAM if SPL
 	imply CMD_DHCP
 	imply CMD_EXT2
 	imply CMD_EXT4