zynq: Add support for U-BOOT SPL

SPL is using ps7_init.c/h files which are generated
from design tools which have to be copied to
boards/xilinx/zynq folder before compilation.

BSS section is moved to SDRAM because fat support
requires more space than SRAM size.

Added:
- MMC and QSPI support
- Boot OS directly from SPL
- Enable SPL command

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>