Implemeted with a Galois LFSR for speed and flexibility compared to Fibonacci version.
Aliases for three popular PRBS' are added for convenience - PRBS9, PRBS15 and PRBS64.
Note that we can't test PRBS64 completely, because the sequence is too long to
be generated.
Change-Id: Ib5331ba5d0b5819929541686fdd87905e2177b74
Alexander Chemeris