powerpc/476: Workaround for PLB6 hang

The 476FP core may hang if an instruction fetch happens during an msync
following a tlbsync.  This workaround makes sure that enough instruction
cache lines are pre-fetched before executing the msync.  (sync and msync
are the same to the compiler.)

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>