docs: driver-model: platform: update the definition of platform_driver

Update the documented struct platform_driver to match the code.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240711200421.11428-1-ebiggers@kernel.org