microblaze: Export xmb_manager functions

When TMR_MANAGER is enabled as module there is a need to export functions
which are present in architecture code.

It has been found by running:
make W=1 C=1 allmodconfig
sed -i -e 's/WERROR=y/WERROR=n/g' .config
make C=1 W=1

which errors out like this:
ERROR: modpost: "xmb_manager_register" [drivers/misc/xilinx_tmr_manager.ko] undefined!
ERROR: modpost: "xmb_inject_err" [drivers/misc/xilinx_tmr_inject.ko] undefined!

Fixes: a5e3aaa6 ("microblaze: Add xmb_manager_register function")
Reported-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/e322dbbbde0feef83f44304ea13249d365d1dc5f.1718799090.git.michal.simek@amd.com