arm64: Use of_get_cpu_hwid()

Replace the open coded parsing of CPU nodes' 'reg' property with
of_get_cpu_hwid().

This change drops an error message for missing 'reg' property, but that
should not be necessary as the DT tools will ensure 'reg' is present.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20211006164332.1981454-5-robh@kernel.org