[U-Boot] Adds driver for Xilinx' xps_spi SPI controller

Adds a driver for Xilinx' xps_spi controller. This code differs in only trivial ways from the altera_spi driver. It plays nice with Thomas Chou's mmc_spi driver, as well as with SPI flash.

Documentation for the SPI core is available here:

http://www.xilinx.com/support/documentation/ip_documentation/xps_spi.pdf