refscale: Allow CPU hotplug to be enabled

It is no longer possible to disable CPU hotplug in many configurations,
which means that the CONFIG_HOTPLUG_CPU=n lines in refscale's Kconfig
options are just a source of useless diagnostics.  In addition, refscale
doesn't do CPU-hotplug operations in any case.  This commit therefore
changes these lines to read CONFIG_HOTPLUG_CPU=y.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>