drivers: rng: optee_rng: register to CONFIG_OPTEE_SERVICE_DISCOVERY

Changes optee_rng driver to register itself has a OP-TEE service so
that a device is bound for the driver when OP-TEE enumerates the
PTA RNG service.

Cc: Sughosh Ganu <sughosh.ganu@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/drivers/rng/Kconfig b/drivers/rng/Kconfig
index 1614368..5dcf681 100644
--- a/drivers/rng/Kconfig
+++ b/drivers/rng/Kconfig
@@ -41,6 +41,7 @@
 config RNG_OPTEE
 	bool "OP-TEE based Random Number Generator support"
 	depends on DM_RNG && OPTEE
+	default y if OPTEE_SERVICE_DISCOVERY
 	help
 	  This driver provides support for the OP-TEE based Random Number
 	  Generator on ARM SoCs where hardware entropy sources are not