ARM: keystone: Move CPU bringup code to dedicated asm file

Because of inline asm usage in platsmp.c, smc instruction
creates build failure for ARM V6+V7 build where as using instruction
encoding for smc breaks the thumb2 build.

So move the code snippet to separate asm file and mark
it with 'armv7-a$(plus_sec)' to avoid any build issues.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>