ARM: broadcom: Use of_get_cpu_hwid()

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

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211006164332.1981454-4-robh@kernel.org