xilinx: Use variable for passing board_name

xilinx: Use variable for passing board_name

Use variable which points to DEVICE_TREE by default. The reason for this
change is to enable DTB_RESELECT and MULTI_DTB_FIT where board detection
can be used for change DTB at run time. That's why there must be reference
in board_fit_config_name_match() via variable instead of hardcoding it
which is sufficient for that use case.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>