arm64: mm: update max_dma32 before calculating size of NORMAL zone

Commit f483a853 ("arm64: mm: fix booting on systems with no memory
below 4GB") sets max_dma32 to the minimum of the maximum pfn and
MAX_DMA32_PFN. This value is later used as the base of the NORMAL zone,
which is incorrect when MAX_DMA32_PFN is below the minimum pfn (i.e. all
memory is above 4GB).

This patch fixes the problem by ensuring that max_dma32 is always set to
the end of the DMA32 zone.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>