of: Move CPU node related functions to their own file

of: Move CPU node related functions to their own file

drivers/of/base.c is quite long and we've accumulated a number of CPU
node functions. Let's move them to a new file, cpu.c, along with the
lone of_cpu_device_node_get() in of_device.h. Moving the declaration has
no effect yet as of.h is included by of_device.h. This serves as
preparation to disentangle the includes in of_device.h and
of_platform.h.

Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-4-581e2605fe47@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>