usb: Export BOS descriptor to sysfs

Motivation
----------

The binary device object store (BOS) of a USB device consists of the BOS
descriptor followed by a set of device capability descriptors. One that is
of interest to users is the platform descriptor. This contains a 128-bit
UUID and arbitrary data, and it allows parties outside of USB-IF to add
additional metadata about a USB device in a standards-compliant manner.
Notable examples include the WebUSB and Microsoft OS 2.0 descriptors.

The kernel already retrieves and caches the BOS from USB devices if its
bcdUSB is >= 0x0201. Because the BOS is flexible and extensible, we export
the entire BOS to sysfs so users can retrieve whatever device capabilities
they desire, without requiring USB I/O or elevated permissions.

Implementation
--------------

Add bos_descriptors attribute to sysfs. This is a binary file and it works
the same way as the existing descriptors attribute. The file exists only if
the BOS is present in the USB device.

Also create a binary attribute group, so the driver core can handle the
creation of both the descriptors and bos_descriptors attributes in sysfs.

Signed-off-by: Elbert Mai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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