spi: microblaze: Adds driver for Xilinx SPI controller

spi: microblaze: Adds driver for Xilinx SPI controller

This is an improved version of the driver patch original
submitted by Graeme Smecher <graeme.smecher@mail.mcgill.ca>

The changes are:
    - remove hard coded Xilinx BSP defines (XPAR_SPI_*) and
      use CONFIG_SYS_SPI_BASE from config.h instead
    - add extensive register struct definitions
    - remove offset calculation for register access and
      use the new register struct instead
    - move default SPI controller configuration from
      spi_setup_slave() to spi_claim_bus()
    - add spi_set_speed()
    - insert SPI controller deactivation in spi_release_bus()
    - protect while loops in spi_xfer() with counter / timeouts
    - support SPI mode flags: LSB_FIRST, CPHA, CPOL, LOOP

Come from:
    http://patchwork.ozlabs.org/patch/71797/

Signed-off-by: Stephan Linz <linz@li-pro.net>
Tested-by: Michal Simek <monstr@monstr.eu>