arm64: dts: exynos: Add initial Exynos850 SoC support

Samsung Exynos850 is ARMv8-based mobile-oriented SoC. This patch adds
initial SoC support. It's not comprehensive yet, some more devices will
be added later. Right now only crucial system components and most needed
platform devices are defined.

Crucial features (needed to boot Linux up to shell with serial console):

  * Octa cores (Cortex-A55), supporting PSCI v1.0
  * ARM architected timer (armv8-timer)
  * Interrupt controller (GIC-400)
  * Pinctrl nodes for GPIO
  * Serial node

Basic platform features:

  * Clock controller CMUs
  * OSCCLK clock
  * MCT timer
  * ARM PMU (Performance Monitor Unit)
  * Chip-id
  * RTC
  * Reset
  * Watchdog timers
  * eMMC
  * I2C
  * HSI2C
  * USI

All those features were already enabled and tested on E850-96 board with
minimal BusyBox rootfs.

Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20211221142417.19312-2-semen.protsenko@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>