arm64: dts: broadcom: bcm4908: describe PMB block

PMB (Power Management Bus) controls powering connected devices (e.g.
PCIe, USB, SATA). In BCM4908 it's a part of the PROCMON block.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>