[POWERPC] i8259: Add disable method

Since commit 76d21601

, the NE2000 card
is not working anymore on PPC and POWERPC and produces WATCHDOG
timeouts.

The patch below fixes that the same way it has been done on x86, x86_64
and MIPS.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>