memory: of: parse max-freq property

Passing the memory timings maximum frequency as an unit address was
a workaround and instead 'max-freq' is preferred.  Look for 'max-freq'
first and then fallback to 'reg'.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alim Akhtar <alim.ahtar@samsung.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220206135807.211767-8-krzysztof.kozlowski@canonical.com