ARM: BCM53573: Specify PMU and its ILP clock in the DT

ILP clock (sometimes called a "slow clock") is a part of PMU (Power
Management Unit). There has been recently added a driver for it, so add
a proper entry in the DT as well.

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