Safe API to enable the ITM

The ITM can easily be enabled using OpenOCD but that requires having a debugger connected to the MCU. ITM can be used for logging or as a communication channel in a production application; in that case the MCU must take care of initializing the ITM. Right now enabling the ITM requires unsafe code; this is what it looks like:

    unsafe {
        // enable TPIU and ITM
        cp.DCB.demcr.modify(|r| r | (1 << 24));

        // prescaler
        let swo_freq = 2_000_000;
        cp.TPIU.acpr.write((clocks.sysclk().0 / swo_freq) - 1);

        // SWO NRZ
        cp.TPIU.sppr.write(2);

        cp.TPIU.ffcr.modify(|r| r & !(1 << 1));

        // STM32 specific: enable tracing in the DBGMCU_CR register
        const DBGMCU_CR: *mut u32 = 0xe0042004 as *mut u32;
        let r = ptr::read_volatile(DBGMCU_CR);
        ptr::write_volatile(DBGMCU_CR, r | (1 << 5));

        // unlock the ITM
        cp.ITM.lar.write(0xC5ACCE55);

        cp.ITM.tcr.write(
            (0b000001 << 16) | // TraceBusID
            (1 << 3) | // enable SWO output
            (1 << 0), // enable the ITM
        );

        // enable stimulus port 0
        cp.ITM.ter[0].write(1);
    }

We should have a safe API to enable the ITM. The API could be an ITM::enable(&mut self, prescaler: u32) method to globally enable the ITM and an ITM::{enable,disable}_port(&mut self, i: u8) to enable / disable an individual port.

Start with the ITM, DCB, and TPIU register definitions and compare them with the unsafe initialization sequence in the issue. Done means the crate exposes safe methods to enable ITM, set its prescaler, and enable or disable individual stimulus ports without requiring callers to write unsafe code.