Add support for configuring pin for w1-gpio kernel module

Patch implement runtime configuration option for setting GPIO pin like 1-Wire master device.

When use module you may use it on follow way :
modprobe w1-gpio (standard hard coded pin 4 will be used)
modprobe w1-gpio gpiopin=18 (pin 18 will be used for 1-Wire devices. Pin number need to be real pin number not pin position on GPIO interface)

More about issue may be found on http://www.raspberrypi.org/phpBB3/viewtopic.php?f=37&t=40044