Use GPIO pin 23 for PPS input

Anybody watching this repository may have noticed that the pin
previously used was pin 18, closest to the TX/RX pin for GPS
NMEA I/O. However, that pin is now being used by PWM driver.

The modifications done in these and previous commits are
sourced from: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=9&t=1970

backported from https://github.com/davidk/adafruit-raspberrypi-linux-pps
commit 6621da156da0bd49dea7bf9aca4e06b83c6c5dfb
Signed-off-by: Steve Conklin <steve@conklinhouse.com>