boards: up_squared: Rework dts_fixup.h to work w/o BOARD_DIR include

Merge up_squared_dts_fixup.h and up_squared_sbl_dts_fixup.h into
dts_fixup.h.  This allows us to remove BOARD_DIR from being included
everywhere.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>