Adds standard DONT_CARE pin name for specifying that any pin will work

This patch dds a standard DONT_CARE pin name that allows specifying that any pin will work in a pin map.  This is currently used in the nRF51822 serial API to allow specifying any pins as a UART.

On a chip like the nRF51822 that supports mapping functions to any arbitrary pin, it seems overly restrictive to limit a user to only use a set of pin mappings.

In my case I built a custom board that has a USB to UART and another port that could either be a UART or a number of other functions.  I need to be able to map the UART to one of two different pin sets.  This isn't supported by the PinMap in the nRF51822 serial_api.c.

I"m open to suggestions if someone has a better way of implementing this.