altos: Allow STM DMA channels to be reserved for one use

altos: Allow STM DMA channels to be reserved for one use

This allows a single user of a DMA channel to reserve it for use
without needing to lock the mutex; this is required for DMA from the
ADC to work on megametrum as it wants to start DMA from an interrupt
handler, which cannot block on a mutex.

Signed-off-by: Keith Packard <keithp@keithp.com>