[POWERPC] Remove interrupt-controller as a property under /chosen

 Remove interrupt-controller as a valid property under /chosen in
 the documentation.  There is a consensus that an
 interrupt-controller property does not belong under /chosen.
 /chosen is specifically for dynamic properties set at runtime.

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>