powerpc/cacheinfo: Set pr_fmt()

Set pr_fmt() so we get a nice prefix on messages.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190627051537.7298-2-nathanl@linux.ibm.com