Allow USB to work with V2 clocking API

[rnd-ash]

Summary

Continuing my work to allow more peripherals to work with the V2 clocking API, this PR adds a new method (Maybe name should be changed to something more permanent), that allows the UsbBus to be constructed with the new V2 clocking API.

I've left the original constructor untouched in order to maintain compatibility.

Now, in the BSPs, USB can be constructed like so:

pub fn usb(
    usb: pac::Usb,
    pclk_usb: Pclk<Usb, Gclk0Id>, // Can use any Gclk here
    mclk: &mut pac::Mclk,
    d_minus: impl Into<UsbDm>,
    d_plus: impl Into<UsbDp>,
) -> UsbBusAllocator<UsbBus> {
    UsbBusAllocator::new(UsbBus::new_with_v2_clock(pclk_usb, mclk, d_minus.into(), d_plus.into(), usb))
}

[rnd-ash]

Additional note. I'm not sure why we even need the pclk here, since the original constructor never made use of the UsbClock that it was given. Maybe we can remove the clock entirely from the V1 constructor and then it would work with both clocking APIs?