Add support for more than one SPI bus [imported]

This is [Issue 774](http://code.google.com/p/arduino/issues/detail?id=774) moved from a Google Code project.
Added by 2012-01-05T07:11:07.000Z by [rmackay...@gmail.com](http://code.google.com/u/102217401446260941157/).
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Enhancement, Priority-Medium, Component-Core
### Original description

**What change would like to see?**
Atmega 1280s, 2560s support multiple SPI buses and it would be very handy to be able to use them.

**Why?**
I'm on the arducopter team and in the APM2 hardware we use one SPI bus for the dataflash, and another for the MPU6000 (sensor that provides 3d accelerometer, gyro and magnetometer) and barometer.

Would this cause any incompatibilities with previous versions? If so, how can these be mitigated?
Yes, it might in that I believe you lose a serial port if you use it as an SPI. This means we might need to be smart about whether one of the serial ports (Serial3?) or SPI2 exists because both couldn't be used at the same time.