arm: exynos: Enable TRNG on E850-96 board

arm: exynos: Enable TRNG on E850-96 board

Enable True Random Number Generator (TRNG) on E850-96 board. To do so:
  1. Enable DM_RNG and RNG_EXYNOS for TARGET_E850_96
  2. Add TRNG node to E850-96 device tree
  3. Enable 'rng' command support for easy TRNG testing

TRNG node is already applied in Linux kernel device tree, but it hasn't
appeared in upstream dts yet. Add it in U-Boot override dtsi file
temporarily; it can be removed once it appears in upstream dts.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>