powerpc: Fix style of self-test config prompts

A few of the config prompts for powerpc self-tests have periods at the
end, which is inconsistent with the rest of the prompts. Remove the
periods.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>