[POWERPC] ps3: fix interrupt bmp

Add a comment and a preprocessor macro to help clearify the alignment
needs of the PS3 interrupt bitmap.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>