zynqmp: spl: support DRAM ECC initialization

zynqmp: spl: support DRAM ECC initialization

Use the ZDMA channel 0 to initialize the DRAM banks. This avoid
spurious ECC errors that can occur when accessing unitialized memory.

The feature is enabled by setting the option
CONFIG_SPL_ZYNQMP_DRAM_ECC_INIT and providing the following data:

 SPL_ZYNQMP_DRAM_BANK1_BASE: start of memory to initialize
 SPL_ZYNQMP_DRAM_BANK1_LEN : len of memory to initialize (hex)
 SPL_ZYNQMP_DRAM_BANK2_BASE: start of memory to initialize
 SPL_ZYNQMP_DRAM_BANK2_LEN : len of memory to initialize (hex)

Setting SPL_ZYNQMP_DRAM_BANK_LEN to 0 takes no action.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>