arm: zynq: Add Kconfig option for any DDR specific initialization

Add Kconfig option for ddr init as this might be required
in cases like ddr less systems where we want to skip ddrc
init and this option is useful for it.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>