configs: sandbox: Enable random number generator(rng) device

Enable support for random number generator on sandbox configs. This is
aimed primarily at adding unit test support for rng uclass.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

configs/sandbox64_defconfig

@@ -161,6 +161,8 @@ CONFIG_REGULATOR_RK8XX=y
 CONFIG_REGULATOR_S5M8767=y
 CONFIG_DM_REGULATOR_SANDBOX=y
 CONFIG_REGULATOR_TPS65090=y
+CONFIG_DM_RNG=y
+CONFIG_RNG_SANDBOX=y
 CONFIG_DM_PWM=y
 CONFIG_PWM_SANDBOX=y
 CONFIG_RAM=y

configs/sandbox_defconfig

@@ -181,6 +181,8 @@ CONFIG_REGULATOR_RK8XX=y
 CONFIG_REGULATOR_S5M8767=y
 CONFIG_DM_REGULATOR_SANDBOX=y
 CONFIG_REGULATOR_TPS65090=y
+CONFIG_DM_RNG=y
+CONFIG_RNG_SANDBOX=y
 CONFIG_DM_PWM=y
 CONFIG_PWM_SANDBOX=y
 CONFIG_RAM=y