usb: dwc2: add bus suspend/resume for dwc2

Hcd controller needs bus_suspend/resume, dwc2 controller make
root hub generate suspend/resume signal with hprt0 register
when work in host mode.
After the root hub enter suspend, we can make controller enter
low power state with PCGCTL register.

We also update the lx_state for hsotg state.

This patch has tested on rk3288 with suspend/resume.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>