microblaze: start.S: use stack space as scratch memory for endian offset

To simpify the code, use stack space as scratch memory for endian offset
calculation, rather than saving/restoring the first unused MB vector.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211130163358.2531677-5-ovidiu.panait@windriver.com


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