feat(dma): implement type-safe DMA request source API

Replace raw u8 DMA request sources with typed request system
following the embassy-imxrt pattern. This provides compile-time
verification that peripherals use correct DMA request sources.

Changes:
- Define DmaRequest trait for type-safe request sources
- Add sealed SealedDmaRequest trait to prevent external implementation
- Create Lpuart{0-5}{Tx,Rx}Request marker types for each LPUART DMA request
- Update DmaChannel::set_request_source() to accept generic R: DmaRequest
- Replace Instance::TX_DMA_REQ/RX_DMA_REQ u8 constants with associated types
- Update all LPUART DMA call sites to use typed request API

This prevents errors like using LPUART2_RX request for LPUART0
at compile time with zero runtime overhead.

Addresses felipebalbi's review comment on PR #52 line 86.

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