arm64: dts: exynos: Add initial E850-96 board support

arm64: dts: exynos: Add initial E850-96 board support

E850-96 is a 96boards development board manufactured by WinLink. It
incorporates Samsung Exynos850 SoC, and is compatible with 96boards
mezzanine boards [1], as it follows 96boards standards.

This patch adds minimal support for E850-96 board. Next features are
enabled in board dts file and verified with minimal BusyBox rootfs:

 * User buttons
 * LEDs
 * Serial console
 * Watchdog timers
 * RTC
 * eMMC

[1] https://www.96boards.org/products/mezzanine/

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20211221142417.19312-3-semen.protsenko@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>