selftests/powerpc: Allow choice of CI memory location in alignment_handler test

The alignment handler selftest needs cache-inhibited memory and
currently /dev/fb0 is relied on to provided this. This prevents running
the test on systems without /dev/fb0 (e.g., mambo). Read the commandline
arguments for an optional path to be used instead, as well as an
optional offset to be for mmaping this path.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Tested-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200520021103.19798-1-jniethe5@gmail.com