devicetree: Fix typo in power9-phb4.dts
devicetree: Fix typo in power9-phb4.dts

Change "impi" to "ipmi".

Signed-off-by: Reza Arbab <4c896a97e6b998365c2d50a267b0f1ca5f7995de@linux.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

legoater/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,legoater/skiboot,qemu/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot