cpu: Hide cpu_up/down

Provide a special exported function for the device core to bring a cpu
up/down and hide the real cpu_up/down as they are treated as private
functions. cpu_up/down are lower level API and users outside the cpu
subsystem should use add/remove_cpu() which will take care of extra
housekeeping work like keeping sysfs in sync.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: "Rafael J. Wysocki" <rafael@kernel.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Josh Poimboeuf <jpoimboe@redhat.com>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: "Peter Zijlstra (Intel)" <peterz@infradead.org>
CC: Jiri Kosina <jkosina@suse.cz>
CC: Daniel Lezcano <daniel.lezcano@linaro.org>
CC: Eiichi Tsukata <devel@etsukata.com>
CC: Zhenzhong Duan <zhenzhong.duan@oracle.com>
CC: Ingo Molnar <mingo@kernel.org>
CC: Pavankumar Kondeti <pkondeti@codeaurora.org>
CC: linux-kernel@vger.kernel.org