stm32mp1: rng: Add a driver for random number generator(rng) device

Add a driver for the rng device found on stm32mp1 platforms. The
driver provides a routine for reading the random number seed from the
hardware device.

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>
Remove a superfluous blank line
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>