Add support for mig_7series memory controller

For Microblaze in Vivado.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Enable to select text_base address on eram_base

text_base address can be equal to eram_base.
It just means that u-boot should be added to
the start of ram.

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