Add xon flow control when FIFO is almost full at block level.

libraries/base/dp/src/vhdl/dp_fifo_core.vhd

libraries/base/dp/src/vhdl/dp_fifo_core_arr.vhd

libraries/base/dp/src/vhdl/dp_fifo_dc.vhd

libraries/base/dp/src/vhdl/dp_fifo_dc_arr.vhd

libraries/base/dp/src/vhdl/dp_fifo_dc_mixed_widths.vhd

libraries/base/dp/src/vhdl/dp_fifo_fill_core.vhd

libraries/base/dp/src/vhdl/dp_fifo_fill_eop.vhd

libraries/base/dp/src/vhdl/dp_fifo_fill_sc.vhd

libraries/base/dp/src/vhdl/dp_fifo_sc.vhd