arm64: mm: Fix unused variable warning in zone_sizes_init

When building arm64 allnoconfig, CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32
get disabled so there is a warning about max_dma being unused.

../arch/arm64/mm/init.c:215:16: warning: unused variable 'max_dma'
[-Wunused-variable]
        unsigned long max_dma = min;
                      ^
1 warning generated.

Add __maybe_unused to make this clear to the compiler.

Fixes: 1a8e1cef ("arm64: use both ZONE_DMA and ZONE_DMA32")
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>