[r2dbe_vdif_lib] added register to pass enable on pps

tied the enable signal to a register that will only pass the value
when fired by the pps.

also added registers to monitor the FIFO outputs in case issues
of delay persists

by compiling the top level, we see these delay issues removed
(_Aug_06_1943.bof.gz)
[r2dbe_top, r2dbe_vdif] added little endian swap

added software register inside the mk_tengbe_sigs block
and added ability to do a little endian byte swap.