microblaze: heartbeat: fix missing prom.h include

Since commit 96f0e6fc ("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: 96f0e6fc ("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>