SoC: exynos: add support for exynos 78x0

Samsung Exynos 7880 \ 7870 - SoC for mainstream smartphones and tablets
introduced on March 2017.
Features:
- 8 Cortex A53 cores
- ARM Mali-T830 MP3 GPU
- LTE Cat. 7 (7880) or 6 (7870) modem

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>