[PARISC] Add sync required after fdc to enforce insn ordering

PA20 arch book (page 7-52 and 7-55) indicate a "sync" is required after
the FDC "to enforce instruction ordering". And we want to make
sure FIC is executed after FDC has retired.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>