doc: migration-guide-3.5: LPC DMA dma-channels

Include in migration guide about how the meaning of dma-channels
to the LPC DMA driver has been changed/fixed, which will
affect downstream boards, and information about the new Kconfig
achieving the old functionality of dma-channels.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>