powerpc/sstep: Add testcases for VSX vector paired load/store instructions

Add testcases for VSX vector paired load/store instructions.
Sample o/p:

  emulate_step_test: lxvp           : PASS
  emulate_step_test: stxvp          : PASS
  emulate_step_test: lxvpx          : PASS
  emulate_step_test: stxvpx         : PASS
  emulate_step_test: plxvp          : PASS
  emulate_step_test: pstxvp         : PASS

Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201011050908.72173-6-ravi.bangoria@linux.ibm.com