SoC have internal I/O buses that can't be proved for devices. The
devices on the buses can be accessed directly without additinal
configuration required. This type of bus is represented as
"simple-bus". In some platforms, we name "soc" with "simple-bus"
attribute and many devices are hooked under it described in DT
(device tree).

In commit bf74ad5b ("Hold the device's parent's lock during
probe and remove") to solve USB subsystem lock sequence since
USB device's characteristic. Thus "soc" needs to be locked
whenever a device and driver's probing happen under "soc" bus.
During this period, an async driver tries to probe a device which
is under the "soc" bus would be blocked until previous driver
finish the probing and release "soc" lock. And the next probing
under the "soc" bus need to wait for async finish. Because of
that, driver's async probe for init time improvement will be
shadowed.

Since many devices don't have USB devices' characteristic, they
actually don't need parent's lock. Thus, we introduce a lock flag
in bus_type struct and driver core would lock the parent lock base
on the flag. For USB, we set this flag in usb_bus_type to keep
original lock behavior in driver core.

Async probe could have more benefit after this patch.
Signed-off-by: Martin Liu <liumartin@google.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>