ARM: BCM53573: Specify ports for USB LED for Tenda AC9

This uses trigger-sources documented in commit 80dc6e1c

 ("dt-bindings:
leds: document new trigger-sources property") to specify USB ports. Such an
information can be used by operating system to setup LEDs behavior.

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