selftests/powerpc: Fix pkey access right updates

The Power ISA mandates that all writes to the Authority
Mask Register (AMR) must always be preceded as well as
succeeded by a context synchronizing instruction.

This makes sure that the tests follow this requirement
when attempting to update a pkey's access rights.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200604125610.649668-2-sandipan@linux.ibm.com