ARM: dts: BCM53573: Specify USB ports of on-SoC controllers

Broadcom OHCI and EHCI controllers always have 2 ports each on the root
hub. Describe them in DT to allow specifying extra info or referencing
port nodes.

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