arm: exynos: odroid: Fix build if BOARD_TYPES are not set

arm: exynos: odroid: Fix build if BOARD_TYPES are not set

CONFIG_BOARD_TYPES is necessary for Odroid X/X2/U3 boards to detect
proper revision.  However building should succeed even without it.
While moving code around, document also the reference clock selection.

This fixes the build error without CONFIG_BOARD_TYPES:

    board/samsung/odroid/odroid.c: In function 'board_usb_init':
    board/samsung/odroid/odroid.c:473:8: error: 'gd_t' {aka 'volatile struct global_data'} has no member named 'board_type'
      if (gd->board_type == ODROID_TYPE_U3)
            ^~

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>