clk: stm32mp1: Add a clock entry for RNG1 device

Add an entry for allowing clock enablement for the random number
generator peripheral, RNG1.

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