Commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for
platforms with no DMA memory zones") introduced different definitions
for 'arm64_dma_phys_limit' depending on CONFIG_ZONE_DMA{,32} based on
a late suggestion from Pasha. Sadly, this results in a build error when
passing W=1:
| arch/arm64/mm/init.c:90:19: error: conflicting type qualifiers for 'arm64_dma_phys_limit'
Drop the 'const' for now and use '__ro_after_init' consistently.