libopencmsis: Fill in missing CMSIS interrupt links

And correct a minor typo in the generated code.
stm32/flash: Make comments match reality

Copy/paste commentary with wrong sizes.
stm32/dac: Follow ref-man guidelines on GPIO config

GPIO pins should be configured for Analog IN on virtually all families
to reduce parasitic consumption.

Reported-by: Roger Wolff <R.E.Wolff@BitWizard.nl>
usb: Support for MIDI class devices

Provides all of the config descriptors for MIDI class devices together
with a sub-set of the audio class shufficient to put together a full
fledged MIDI device.

Also requires is an extension to struct usb_endpoint_descriptor to allow
class specific endpoint descriptors to be expressed. All of
libopencm3-examples use the USB_DT_ENDPOINT_SIZE (rather then sizeof on
the structure) so there should be no compatibility problems with this
change.