MIPS: JZ4770: DMA: Place EXPORT_SYMBOL after function it exports

This is the conventional location for it.
MIPS: JZ4770: DMA: Removed test code
MIPS: JZ4770: DMA: Clean up descriptor types

Removed two types that were unused.
Removed typedef from the third type that is used.
mmc: JZ4770: Use standard name for DMA descriptor struct

Instead of using a macro, use the name directly, including "struct".
This is required now that the platform header no longer has a typedef.

Also moved inclusion of the DMA header to jz_mmc_dma.c, since that is
the only source file that actually has to know about DMA.
MIPS: JZ4770: DMA: Removed unused functions
MIPS: JZ4770: DMA: Made get_dma_chan() and its table private

There were no callers outside of arch/mips/jz4770/dma.c.
MIPS: JZ4770: DMA: Moved macros used in dma_dev_table to dma.c

They weren't used elsewhere, no need to have them in the public header.