microblaze: Rename global function heartbeat()

microblaze:allmodconfig complains for some configurations that 'heartbeat'
is redefined as different kind of symbol. This is seen in test compiles
of watchdog drivers, which often use 'heartbeat' as ststic variable.

Since 'heartbeat' is an unfortunate name for a global function,
rename it to microblaze_heartbeat. Also rename the setup function
to microblaze_setup_heartbeat.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>