scripts: Show source max PDP value

Add support for reading the source max PDP values for each port. Print
these in rtk_utils.print_config().

Add unit testing.

BUG=None
TEST=./run_tests.sh
TEST=./rtk_fw_config.py image.bin

Change-Id: Id0caa23012cab9621c5d507797efff740fba5a14
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/pdc/+/7170388
Reviewed-by: Keith Short <keithshort@chromium.org>
Tested-by: Tristan Honscheid <honscheid@google.com>
Commit-Queue: Tristan Honscheid <honscheid@google.com>