drivers/rng: add Amlogic hardware RNG driver

Add support for the hardware random number generator of Amlogic SOCs.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Sughosh Ganu <sughosh.ganu@linaro.org>