arm64: smp: move the pen to a header file

Move the secondary_pen_release variable and the secondary_holding_pen
entry function to asm/smp_plat.h so that the other cpu ops implementations
can share them.

Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>
Signed-off-by: Kumar Gala <galak@codeaurora.org>