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.

Change-Id: I9cee3e01185df5e9977795cc778848d91d370b70
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>