microblaze: Wire up SPI driver

Depending on XILINX_SPI_FLASH_BASEADDR enable SPI flash
and environment in SPI flash.

Expected values from xparameters.h are:
    - XILINX_SPI_FLASH_BASEADDR
    - XILINX_SPI_FLASH_MAX_FREQ
    - XILINX_SPI_FLASH_CS

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