sandbox: rng: Add a random number generator(rng) driver

Add a sandbox driver for random number generation. Mostly aimed at
providing a unit test for rng uclass.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/rng/Kconfig b/drivers/rng/Kconfig
index c9c4751..35a3bd1 100644
--- a/drivers/rng/Kconfig
+++ b/drivers/rng/Kconfig
@@ -6,6 +6,14 @@
 	  This interface is used to initialise the rng device and to
 	  read the random seed from the device.
 
+config RNG_SANDBOX
+	bool "Sandbox random number generator"
+	depends on SANDBOX && DM_RNG
+	select CONFIG_LIB_RAND
+	help
+	  Enable random number generator for sandbox. This is an
+	  emulation of a rng device.
+
 config RNG_STM32MP1
 	bool "Enable random number generator for STM32MP1"
 	depends on ARCH_STM32MP && DM_RNG