selftests/powerpc: Fix typos

Fix two typos in the file header. Replacing the word 'priviledged'
by 'privileged' and 'exuecuted' by 'executed'.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Gustavo Romero <gromero@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>