microblaze: migrate CONFIG_SYS_USR_EXCEP to Kconfig

Migrate CONFIG_SYS_USR_EXCEP to Kconfig. Also, rename it to
XILINX_MICROBLAZE0_USR_EXCEP in order to match the naming convention of
microblaze-generic Kconfig options.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211130163358.2531677-7-ovidiu.panait@windriver.com


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