[POWERPC] Add correct interrupt property for pegasos ide

The firmware assigns irq 20/21 to the VIA IDE device on Pegasos.
But the required interrupt is 14/15.
Maybe someone confused decimal vs. hexadecimal values.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>