serial: s5p: Add Exynos850 compatible

Enable serial support for Exynos850 SoC by adding the corresponding
compatible string. No additional changes needed, the driver works as is
on Exynos850. Related USI and PMU configuration is enabled in separate
drivers. The only other dependencies are clock and pinctrl drivers,
which are already enabled too.

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