usb: ulpi: Add debugfs support

This adds a debugfs file for ULPI devices which contains a dump of their
registers. This is useful for debugging basic connectivity problems. The
file is created in ulpi_register because many devices will never have a
driver bound (as they are managed in hardware by the USB controller
device).

The root directory of this subsystem is created before we register the
bus to ensure that devices can always create their directories.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20220127190004.1446909-4-sean.anderson@seco.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>