ARM: zynq: Specify MMC controller number for boot sequence

Xilinx Zynq SoC has two sdhci controllers but boot is only possible from
the first one. That's why there is a need to specify controller number.
mmc1 is supposed to be secondary boot device and should be also listed in
distribution boot.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>