PNP: Add helper macro for pnp_register_driver boilerplate

This patch introduces the module_pnp_driver macro which is a
convenience macro for PNP driver modules similar to module_pci_driver.
It is intended to be used by drivers which init/exit section does nothing
but register/unregister the PNP driver. By using this macro it is
possible to eliminate a few lines of boilerplate code per PNP driver.

Based on work done by Lars-Peter Clausen <lars@metafoo.de> for other
busses (i2c and spi) and Greg KH for PCI.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>