arm: zynq: Add mini u-boot configuration for zynq

Add configuration files/dtses for mini u-boot configurations which runs
out of OCM.

ram top is calculated from 0 that's why +#define CONFIG_SYS_SDRAM_BASE
0xfffc0000
+#define CONFIG_SYS_SDRAM_SIZE  0x40000
was hardcoded.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>