microblaze: start.S: add support for configurable vector base address

microblaze: start.S: add support for configurable vector base address

Current code assumes that the vector base address is always at 0x0.
However, this value is configurable for MicroBlaze, so update the
__setup_exceptions routine to work with any vector base address.

The r4 register is reserved for the vector base address inside
__setup_exceptions and the function prologe/epilogue are also updated to
save and restore r4.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211130163358.2531677-9-ovidiu.panait@windriver.com
Signed-off-by: Michal Simek <michal.simek@xilinx.com>