board: samsung: exynos-mobile: add exynos7870 support and three devices

board: samsung: exynos-mobile: add exynos7870 support and three devices

Add basic support for the Exynos7870 SoC, this includes device tree
match logic using multiple boards, where devices use a stub dtb in
Samsung's QCDT format. S-BOOT, the previous stage bootloader, places its
cmdline arguments there, which has identifying information.

This is added with support for three devices:
 * Samsung Galaxy A2 Core (codename: a2corelte)
 * Samsung Galaxy J6 (codename: j6lte)
 * Samsung Galaxy J7 Prime (codename: on7xelte)

Add their device trees in the defconfig, and also enable the clock and
pin controller drivers.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>