Add PERIPH_A and PERIPH_B to Due pinMode() options

_From @CF20852 on August 24, 2014 16:47_

I wanted to configure TIOA7 to come out on D3 on my Due. I found that I could do so by adding

```
#define PERIPH_A 0x3
#define PERIPH_B 0x4
```

to wiring_constants.h in the hardware/arduino/sam/cores/arduino folder, and

```
case PERIPH_A:
PIO_Configure(
g_APinDescription[ulPin].pPort,
PIO_PERIPH_A,
g_APinDescription[ulPin].ulPin,
g_APinDescription[ulPin].ulPinConfiguration ) ;

break ;

case PERIPH_B:
PIO_Configure(
g_APinDescription[ulPin].pPort,
PIO_PERIPH_B,
g_APinDescription[ulPin].ulPin,
g_APinDescription[ulPin].ulPinConfiguration ) ;

break ;
```

to the switch/case statement in the pinMode code in wiring_digital.c in the same folder.
Then in my sketch I can just write

```
pinMode(3, PERIPH_B);
```

I figure if this is was a good idea, someone would have already done it :-) But here it is, just in case.
And yes, the PERIPH_A stuff is superfluous to my immediate need.

_Copied from original issue: arduino/Arduino#2257_