microblaze: warn if of_iomap() failed

of_iomap() can return NULL. The function is void thus error propagation
is not possible but at least a WARN_ON() seems warranted here as a silent
failure might lead to a hard to understand system state.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>