phylib: make mdio-gpio work without OF (v4)

make mdio-gpio work with non OpenFirmware gpio implementation.

Aditional changes to mdio-gpio:
- use gpio_request() and gpio_free()
- place irq[] array in struct mdio_gpio_info
- add module description, author and license
- add note about compiling this driver as module
- rename mdc and mdio function (were ugly names)
- change MII to MDIO in bus name
- add __init __exit to module (un)loading functions
- probe fails if no phys added to the bus
- kzalloc bitbang with sizeof(*bitbang)

Changes since v3:
- keep bus naming "%x" to be compatible with existing drivers.

Changes since v2:
- more #ifdefs reduction
- platform driver will be registered on OF platforms also
- unified platform and OF bus_id to phy%i

Changes since v1:
- removed NO_IRQ
- reduced #idefs

Laurent, please test this driver under OF.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: David S. Miller <davem@davemloft.net>