arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs()

cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable
callback. A spin lock is used to ensure the hook is registered before
the rest of the callback is executed.

On -RT spin_lock() may sleep. However, all the callees in stop_machine()
are expected to not sleep. Therefore a raw_spin_lock() is required here.

Given this is already done under stop_machine() and the work done under
the lock is quite small, the latency should not increase too much.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>