Function cpu_on 

pub fn cpu_on(target_cpu: usize, entry_point: usize, arg: usize)

Power up a core. This call is used to power up cores that either:

• Have not yet been booted into the calling supervisory software.
• Have been previously powered down with a cpu_off call.

target_cpu contains a copy of the affinity fields of the MPIDR register. entry_point is the physical address of the secondary CPU’s entry point. arg will be passed to the X0 register of the secondary CPU.