[DRIVER MODEL] Add platform_driver

Introduce struct platform_driver.  This allows the platform device
driver methods to be passed a platform_device structure instead of
instead of a plain device structure, and therefore requiring casting
in every platform driver.

We introduce this in such a way that any existing platform drivers
registered directly via driver_register continue to work as before,
thereby allowing a gradual conversion to the new platform_driver
methods.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>