arm: exynos: Migrate E850-96 board to OF_UPSTREAM

arm: exynos: Migrate E850-96 board to OF_UPSTREAM

Use upstream device tree files and bindings. To do so:
 - imply (enable) OF_UPSTREAM option for E850-96 target
 - point DEFAULT_DEVICE_TREE in E850-96 config to upstream dts
 - remove now not needed local dts files, binding docs and headers
 - update MAINTAINERS and board/samsung/e850-96/MAINTAINERS
   correspondingly

Upstream device tree files for Exynos850 SoC and E850-96 board are
pretty much the same as local (removed) ones, so the conversion is
rather straightforward and painless in this case. The appended dts file
(arch/arm/dts/exynos850-e850-96-u-boot.dtsi) stays unchanged.

The only remaining local dt-bindings doc for E850-96 board is
exynos-pmu.yaml. It wasn't removed as it's quite different from Linux
kernel version. Particularly U-Boot local version of exynos-pmu.yaml
describes "samsung,uart-debug-1" property, which is not present in Linux
kernel binding. Later it might be upstreamed to Linux kernel, and once
it's done the U-Boot exynos-pmu.yaml binding can be removed.

No functional change.

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