arm: Move the secondary_startup to __CPUINIT section.

Because of addition 1MB alignment after head.txt section,
the secondary_startup needs to move to 1MB aligned section.

CRs-Fixed: 513919
Change-Id: I332d1e4ec95c877406c6ad3318e5f9a3c85cc10f
Signed-off-by: Venkatesh Yadav Abbarapu <quicvenkat@codeaurora.org>