clk: sunxi: document new A31 USB clock compatible
clk: sunxi: document new A31 USB clock compatible

Support for the USB gates and resets on A31 has been recently added
using a new compatible, so let's document it here.

Signed-off-by: Emilio López <86fa523305b8ad6a8a853c04cfef24a0445908d3@elopez.com.ar>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs