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/Makefile b/drivers/Makefile
index bf73b77..9195daf 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -126,7 +126,7 @@
 obj-$(CONFIG_MACH_PIC32) += ddr/microchip/
 obj-$(CONFIG_FUZZ) += fuzz/
 obj-$(CONFIG_DM_HWSPINLOCK) += hwspinlock/
-obj-$(CONFIG_DM_RNG) += rng/
+obj-$(CONFIG_$(SPL_TPL_)DM_RNG) += rng/
 endif
 
 obj-y += soc/