dm: part: Update test to use mmc2

At present this test sets up a partition table on mmc1. But this is used
by the bootstd tests, so it is not possible to run those after this test
has run, without restarting the Python test harness.

This is inconvenient when running tests repeatedly with 'ut dm'. Move the
test to use mmc2, which is not used by anything.

Signed-off-by: Simon Glass <sjg@chromium.org>