drivers: rng: add smccc trng driver

Adds random number generator driver using Arm SMCCC TRNG interface to
get entropy bytes from secure monitor. The driver registers as an
Arm SMCCC feature driver to allow PSCI driver to bind a device for
when secure monitor exposes RNG support from Arm SMCCC TRNG interface.

Cc: Sughosh Ganu <sughosh.ganu@linaro.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>