MIPS: Malta: Cleanup DMA coherence #ifdefs

DMA coherence is not user-selectable in Kconfig, and Malta selects
CONFIG_DMA_MAYBE_COHERENT which in turn selects CONFIG_DMA_NONCOHERENT.
Remove #ifdefs whose conditions can therefore never be true for Malta.

This removes a significant amount of code from bonito_quirks_setup(),
but the code is duplicated in plat_enable_iocoherency() anyway so we
lose nothing but duplication.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/14188/