arm64: Make arm64_dma32_phys_limit static

This variable is only used in the arch/arm64/mm/init.c file for
ZONE_DMA32 initialisation, no need to expose it.
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/init.c

@@ -63,7 +63,7 @@ EXPORT_SYMBOL(vmemmap);
  * bit addressable memory area.
  */
 phys_addr_t arm64_dma_phys_limit __ro_after_init;
-phys_addr_t arm64_dma32_phys_limit __ro_after_init;
+static phys_addr_t arm64_dma32_phys_limit __ro_after_init;
 
 #ifdef CONFIG_KEXEC_CORE
 /*