common: add a prototype for mach_cpu_init()

Add a new file init.h with the prototype for arch_cpu_init
Add a prototype for mach_cpu_init() to avoid a warning:
no previous prototype for ‘mach_cpu_init’

It is a first step to move all the functions prototype
used during U-Boot initialization (board_f.c / board_r.c)
from common.h to init.h

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>