ITM Stimulus ports need to be accessible with different sizes

The amount of data written out is determined by the size of the write.
Writing a full 32 bit value when you only need 8 for printf() style
substantially reduces the available bandwidth of the SWO

Note: this is an API change for doing 32bit writes.
Old:
    ITM_STIM[stimulus_port] = value
New:
    ITM_STIM32(stimulus_port) = value

This api is much more in common with some of the other registers that
behave this way.  As there's very little (if any) code already using
this API, it's a good time to fix it permanently.
Remove misleading ITM register definitions

ITM_SSPSR is the supported parallel trace size, in _bits_
ITM_CSPSR is in _bits_ as well.  There's really no advantage in even
having these sorts of definitions in libopencm3, as these settings are
normally controlled from the debugger side, not the target itself.