Convert CONFIG_SPL_RAM_DEVICE to defconfig

This converts the following to Kconfig:
  CONFIG_SPL_RAM_DEVICE

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index cba51f5d..ea084f4 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -449,6 +449,15 @@
 	  in drivers/power, drivers/power/pmic and drivers/power/regulator
 	  as part of an SPL build.
 
+config SPL_RAM_DEVICE
+	bool "Support booting from preloaded image in RAM"
+	depends on SPL
+	default y if MICROBLAZE || ARCH_SOCFPGA || TEGRA || ARCH_ZYNQ
+	help
+	  Enable booting of an image already loaded in RAM. The image has to
+	  be already in memory when SPL takes over, e.g. loaded by the boot
+	  ROM.
+
 config SPL_SATA_SUPPORT
 	bool "Support loading from SATA"
 	depends on SPL