ARM: socfpga: mark secondary_trampoline as cpuinit

The socfpga_boot_secondary is correctly marked as __cpuinit but references
secondary_trampoline, which was introduced into the init section, causing
this build warning:

WARNING: vmlinux.o(.cpuinit.text+0xf90): Section mismatch in reference from the function socfpga_boot_secondary() to the variable .init.text:secondary_trampoline
The function __cpuinit socfpga_boot_secondary() references
a variable __init secondary_trampoline.
If secondary_trampoline is only used by socfpga_boot_secondary then
annotate secondary_trampoline with a matching annotation.

The solution is to mark secondary_trampoline as cpuinit as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>