ports/stm: add stereo output via DAC CH2 (PA05/A1) for STM32F405

Enable right-channel audio output on PA05 (DAC_CH2) using DMA1 Stream6
Channel7, triggered by the same TIM6 as the left channel. Mono sources
are duplicated to both channels when right_channel is provided.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>