Added support to reserve/enable a GPIO pin to be used from pps-gpio module

This change enables use of existing pps-gpio module with Raspberry Pi.  This change should be "no-op", as it is disabled by default. It adds two kernel cmdline options:

bcm2708.pps_gpio_enable 
bcm2708.pps_gpio_pin 

When user wants to use pps-gpio module they need to add "bcm2708.pps_gpio_enable=1" into cmdline.txt, this will enable PPS on GPIO 18. If any other GPIO pin is desired it can be selected using the second option...

After system is booted with this option, pps-gpio module can simply be loaded and it will find the designated GPIO pin as a PPS source:

```
[   61.691271] pps_core: LinuxPPS API ver. 1 registered
[   61.691305] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[   61.700274] pps pps0: new PPS source pps-gpio.-1
[   61.700357] pps pps0: Registered IRQ 188 as PPS source
```


This change is based on the patch by Hauke Lampe: https://github.com/lampeh/rpi-misc/blob/master/linux-pps/linux-rpi-pps-gpio-bcm2708.diff