USB: ice40-hcd: Add configuration loading test

Provide a debug facility to power off the bridge chip and load the
configuration file. This will be useful to test the configuration
loading multiple times. The below command needs to be executed
to run this test.

echo -n "config_test" > /sys/kernel/debug/ice40_hcd/command

Change-Id: I22d8d5f87f35cd98dafa4189de34169a95e62a67
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>