Add the SPI driver for am335x.

This driver works in PIO mode for now, interrupts are available only when
FIFO is enabled. The FIFO cannot be used with arbitrary sizes which defeat
its general use.

At some point we can add DMA transfers where the FIFO can be more useful.

Tested on uBMC (microBMC) and BBB.

Sponsored by: Rubicon Communications (Netgate)