microblaze: heartbeat: fix missing prom.h include

Since commit 96f0e6fcc9ad ("microblaze: remove redundant early_printk
support") prom.h was removed and one instance in heartbeat.c remained.
Include of.h as it is the actual header needed.

Fixes: 96f0e6fcc9ad ("microblaze: remove redundant early_printk support")
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/heartbeat.c

@@ -11,10 +11,10 @@
 #include <linux/sched.h>
 #include <linux/sched/loadavg.h>
 #include <linux/io.h>
+#include <linux/of.h>
 
 #include <asm/setup.h>
 #include <asm/page.h>
-#include <asm/prom.h>
 
 static unsigned int base_addr;