drivers: ram: sifive: rename fu540_ddr and add fu740 support

Rename fu540_ddr.c to sifive_ddr.c and add fu740 support

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/ram/sifive/Kconfig b/drivers/ram/sifive/Kconfig
index 08de692..0aaac02 100644
--- a/drivers/ram/sifive/Kconfig
+++ b/drivers/ram/sifive/Kconfig
@@ -5,9 +5,9 @@
 	help
 	  This enables support for ram drivers of SiFive SoCs.
 
-config SIFIVE_FU540_DDR
-	bool "SiFive FU540 DDR driver"
+config SIFIVE_DDR
+	bool "SiFive DDR driver"
 	depends on RAM_SIFIVE
-	default y if TARGET_SIFIVE_UNLEASHED
+	default y if TARGET_SIFIVE_UNLEASHED || TARGET_SIFIVE_UNMATCHED
 	help
-	  This enables DDR support for the platforms based on SiFive FU540 SoC.
+	  This enables DDR support for the platforms based on SiFive SoC.