dm: allow setting driver_data before/during bind

This will allow a driver's bind function to use the driver data. One
example is the Tegra186 GPIO driver, which instantiates child devices
for each of its GPIO ports, yet supports two different HW instances each
with a different set of ports, and identified by the udevice_id .data
field.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

doc/driver-model/README.txt

@@ -606,19 +606,24 @@ methods actually defined.
 
 1. Bind stage
 
-A device and its driver are bound using one of these two methods:
+U-Boot discovers devices using one of these two methods:
 
-   - Scan the U_BOOT_DEVICE() definitions. U-Boot It looks up the
-name specified by each, to find the appropriate driver. It then calls
-device_bind() to create a new device and bind' it to its driver. This will
-call the device's bind() method.
+   - Scan the U_BOOT_DEVICE() definitions. U-Boot looks up the name specified
+by each, to find the appropriate U_BOOT_DRIVER() definition. In this case,
+there is no path by which driver_data may be provided, but the U_BOOT_DEVICE()
+may provide platdata.
 
    - Scan through the device tree definitions. U-Boot looks at top-level
 nodes in the the device tree. It looks at the compatible string in each node
-and uses the of_match part of the U_BOOT_DRIVER() structure to find the
-right driver for each node. It then calls device_bind() to bind the
-newly-created device to its driver (thereby creating a device structure).
-This will also call the device's bind() method.
+and uses the of_match table of the U_BOOT_DRIVER() structure to find the
+right driver for each node. In this case, the of_match table may provide a
+driver_data value, but platdata cannot be provided until later.
+
+For each device that is discovered, U-Boot then calls device_bind() to create a
+new device, initializes various core fields of the device object such as name,
+uclass & driver, initializes any optional fields of the device object that are
+applicable such as of_offset, driver_data & platdata, and finally calls the
+driver's bind() method if one is defined.
 
 At this point all the devices are known, and bound to their drivers. There
 is a 'struct udevice' allocated for all devices. However, nothing has been