microblaze: Cleanup PCI_DRAM_OFFSET handling

PCI_DRAM_OFFSET/pci_dram_offset is 0 all the time and there
is no difference for PCI and !PCI cases.
Also remove the whole code which setup archdata.dma_data
which is completely unused.

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