of/fdt: make memblock maximum physical address arch configurable

When parsing the memory nodes to populate the memblock memory
table, we check against high and low limits and clip any memory
that exceeds either one of them.

However, for arm64, the high limit of (phys_addr_t)~0 is not very
meaningful, since phys_addr_t is 64 bits (i.e., no limit) but there
may be other constraints that limit the memory ranges that we can
support.

So rename MAX_PHYS_ADDR to MAX_MEMBLOCK_ADDR (for clarity) and only
define it if the arch does not supply a definition of its own.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>