Remove extern keyword from function declarations ...

Function declarations implicitly have external linkage so do not
need the extern keyword.

Change-Id: Ia0549786796d8bf5956487e8996450a0b3d79f32

Separate BL functions out of arch.h ...

Move the BL function prototypes out of arch.h and into the
appropriate header files to allow more efficient header file
inclusion. Create new BL private header files where there is no
sensible existing header file.

Change-Id: I45f3e10b72b5d835254a6f25a5e47cf4cfb274c3