microblaze: start.S: Factor out exception setup code to __setup_exceptions

microblaze: start.S: Factor out exception setup code to __setup_exceptions

Currently, the exceptions setup code is duplicated in pre-relocation and
post-relocation init. Factor out this code to __setup_exceptions asm
routine to get rid of the duplication.

__setup_exceptions is called with a relocation offset parameter (r5)
which is set to zero for pre-reloc init and gd->reloc_off for post-reloc
exception setup.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>