drivers/rng: simplify Kconfig

For all sandbox systems with DM_RNG we enable RNG_SANDBOX. So we can simply
set the default to yes.

All rng drivers depend on DM_RNG. Use a single 'if' instead of individual
dependencies. Now 'make menuconfig' shows the individual drivers neatly
indented under the DM_RNG entry.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sughosh Ganu <sughosh.ganu@linaro.org>
diff --git a/drivers/rng/Kconfig b/drivers/rng/Kconfig
index 893b89d..c1aa43b 100644
--- a/drivers/rng/Kconfig
+++ b/drivers/rng/Kconfig
@@ -6,16 +6,21 @@
 	  This interface is used to initialise the rng device and to
 	  read the random seed from the device.
 
+if DM_RNG
+
 config RNG_SANDBOX
 	bool "Sandbox random number generator"
-	depends on SANDBOX && DM_RNG
+	depends on SANDBOX
+	default y
 	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
+	depends on ARCH_STM32MP
 	default n
 	help
 	  Enable STM32MP1 rng driver.
+
+endif