rng: Introduce SPL_DM_RNG

Add SPL variant of DM_RNG so that the DM_RNG can be disabled in SPL
if necessary. This may be necessary due to e.g. size constraints of
the SPL.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/rng/Kconfig b/drivers/rng/Kconfig
index cd72852..5758ae1 100644
--- a/drivers/rng/Kconfig
+++ b/drivers/rng/Kconfig
@@ -6,6 +6,13 @@
 	  This interface is used to initialise the rng device and to
 	  read the random seed from the device.
 
+config SPL_DM_RNG
+	bool "Driver support for Random Number Generator devices in SPL"
+	depends on SPL_DM
+	help
+	  This option is an SPL-variant of the DM_RNG option.
+	  See the help of DM_RNG for details.
+
 if DM_RNG
 
 config RNG_MESON