parisc: Flush ITLB in flush_tlb_all_local() only on split TLB machines

flush_tlb_all_local() flushes the ITLB and DTLB of the CPU.
In case the machine does not have separate ITLBs and DTLBs, use the
alternative functionality to replace the code which flushes the ITLB
with nops while keeping the code which flushes the DTLB.

Signed-off-by: Helge Deller <deller@gmx.de>