board: ti: j721e: Guard functions with right #ifdef to avoid build warnings

board_late_init(), setup_board_eeprom_env() and setup_serial() is
called only under CONFIG_BOARD_LATE_INIT, so guard these functions
with the same. Also, reorder these functions to place it under
single #ifdef

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>