AVR32 and AT91SAM9 both have their own identical definitions of
container_of() taken from the Linux kernel. Move it to common.h so
that all architectures can use it.

container_of() is already used by some drivers, and will be used
extensively by the new and improved SPI API.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>