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>

arch/arm/dts/exynos850-e850-96-u-boot.dtsi

@@ -3,6 +3,17 @@
  * Copyright (c) 2023 Linaro Ltd.
  */
 
+&soc {
+	/* TODO: Remove this node once it appears in upstream dts */
+	trng: rng@12081400 {
+		compatible = "samsung,exynos850-trng";
+		reg = <0x12081400 0x100>;
+		clocks = <&cmu_core CLK_GOUT_SSS_ACLK>,
+			 <&cmu_core CLK_GOUT_SSS_PCLK>;
+		clock-names = "secss", "pclk";
+	};
+};
+
 &pmu_system_controller {
 	bootph-all;
 	samsung,uart-debug-1;

arch/arm/mach-exynos/Kconfig

@@ -250,6 +250,8 @@ config TARGET_E850_96
 	select PINCTRL
 	select PINCTRL_EXYNOS850
 	imply OF_UPSTREAM
+	imply DM_RNG
+	imply RNG_EXYNOS
 
 endchoice
 endif

configs/e850-96_defconfig

@@ -11,6 +11,7 @@ CONFIG_DEFAULT_DEVICE_TREE="exynos/exynos850-e850-96"
 CONFIG_SYS_LOAD_ADDR=0x80000000
 # CONFIG_AUTOBOOT is not set
 # CONFIG_DISPLAY_CPUINFO is not set
+CONFIG_CMD_RNG=y
 # CONFIG_NET is not set
 CONFIG_CLK_EXYNOS850=y
 # CONFIG_MMC is not set