USB hardware page out of date

[JamesH65]

https://www.raspberrypi.org/documentation/hardware/raspberrypi/usb/README.md

Appear to be quite a few bits that are out of date now.

Looks OK now - just some residual mention of the old 100mA USB power limit on 1st gen Pi and hotplug issues.

[JamesH65]

Tweaked and merged.