accel/habanalabs: add parent_device sysfs attribute

The device debugfs directory was modified to be named as the
device-name.
This name is the parent device name, i.e. either the PCI address in case
of an ASIC, or the simulator device name in case of a simulator.

This change makes it more difficult for a user to access the debugfs
directory for a specific accel device, because he can't just use the
accel minor id, but he needs to do more device-dependent operations to
get the device name.

To make it easier to get this name, add a 'parent_device' sysfs
attribute that the user can read using the minor id before accessing
debugfs.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>