microblaze: Fix the heartbeat gpio to be more robust

The device tree handling for the gpio in the heart beat was not handling
the system when there was no gpio and it wasn't working with a newer version
of the gpio core which does not have the is-bidir property.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>