powerpc/prom_init: Fixup missing #size-cells on PowerBook6,7

Similar to the PowerMac3,1, the PowerBook6,7 is missing the #size-cells
property on the i2s node.

Depends-on: commit 045b14ca ("of: WARN on deprecated #address-cells/#size-cells handling")
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
[maddy: added "commit" work in depends-on to avoid checkpatch error]
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/875xmizl6a.fsf@igel.home