microblaze: Unify of setting for SPL_NOR/XIP support

XIP is not enabled in SPL. SPL_NOR is enabled but any macro setting with
using SYS_FLASH_BASE are wrong because it is not aligned with DM.
That's why change these macro and align them with TEXT_BASE macro.
Information should be find at run time based on DT but implementation is
not done yet.

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