dm: usb: Convert USB storage to use driver-model for block devs

Update this code to support CONFIG_BLK. Each USB storage device can have
one or more block devices as children, each one representing a LUN
(logical unit) of the USB device.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Stephen Warren <swarren@nvidia.com>