selftests/powerpc: Make dd output quiet

dd logs info to stderr by default. This info is pointless in the
selftests and makes legitimate issues harder to spot.

Pass the option to silence the info logs. Actual errors would still be
printed.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230228000709.124727-4-bgray@linux.ibm.com