This commit adds additional file with implementation of board
detection code for Odroid-XU3/XU4.

The detection depends on compatible found in fdt:
- "samsung,exynos5" - uses Exynos5 generic code
- "samsung,odroidxu3" - try detect XU3 revision

There are few revisions of Odroid XU3/XU4, each can be detected
by checking the value of channel 9 of built-in ADC:
 Rev   ADC  Board
 0.1     0  XU3 0.1
 0.2   372  XU3 0.2 | XU3L - no DISPLAYPORT
 0.3  1280  XU4 0.1

The detection code depends on the ADC+10% value.

Implementation of functions:
- set_board_type()     - read ADC and set type
- get_board_rev()      - returns board revision: 1..3
- get_board_type()     - returns board type string

Additional functions with return values of bool:
- board_is_generic()   - true if found compatible "samsung,exynos5"
                         but not "samsung,odroidxu3"
- board_is_odroidxu3() - true if found compatible "samsung,odroidxu3"
                     ...