[POWERPC] Sky Cpu and Nexus: check for platform_get_resource retcode

Add adds checking for platform_get_resource() return code to prevent
possible NULL pointer usage.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: Brian Waite <waite@skycomputers.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>