[PATCH] ppc64: Don't panic when early __ioremap fails

Early calls to __ioremap() will panic if the hash insertion fails. This
patch makes them return NULL instead. It happens with some pSeries users
who enabled CONFIG_BOOTX_TEXT. The later is getting an incorrect address
for the fame buffer and the hash insertion fails. With this patch, it
will display an error instead of crashing at boot.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>