ARM: imx: flag failure of of_iomap

 imx_set_aips is assuming that the address returned from of_iomap is
valid which it probably is in the normal case - as the call site
is void error propagation is not possible but never the less at least
a WARN_ON() seems warranted here.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: commit e57e4ab5 ("ARM: i.MX: allow disabling supervisor protect via DT")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>