hw/dma/omap_dma: Fold omap_dma_sys_read() and omap_dma_sys_write() into callers

Now we have removed the DMA 3.2 support, we call omap_dma_sys_read()
only for the single address offset 0x400, and similarly for
omap_dma_sys_write().  The other cases in those functions are DMA
3.2-only and now dead code.  Fold the 0x400 register directly into
the callers, and remove the rest.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20260512203414.3633237-9-peter.maydell@linaro.org