ARM/dma-mapping: Remove CMA code when not built with CMA

The MAX_CMA_AREAS could be set to 0, which would result in code that would
attempt to operate beyond the end of a zero-sized array. If CONFIG_CMA
is disabled, just remove this code entirely. Found when building arm
on GCC 10.x for several defconfigs (e.g. axm55xx_defconfig) under
-Warray-bounds:

arch/arm/mm/dma-mapping.c:396:22: warning: array subscript <unknown> is outside array bounds of 'struct dma_contig_early_reserve[0]' [-Warray-bounds]
  396 |         dma_mmu_remap[dma_mmu_remap_num].size = size;
      |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
arch/arm/mm/dma-mapping.c:389:40: note: while referencing 'dma_mmu_remap'
  389 | static struct dma_contig_early_reserve dma_mmu_remap[MAX_CMA_AREAS] __initdata;
      |                                        ^~~~~~~~~~~~~

Cc: Russell King <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Martin Oliveira <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Hari Bathini <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Reviewed-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]

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