Boot memory layout is specific for a platform, but should not be
mixed up with other platform specific attributes. A separate file is
much cleaner and better to compare with other platforms. Take a look
at plat/poplar where it is done the same way.

Moved hikey_def.h to system include folder and moved includes from
hikey_def.h to more general platform_def.h.
Signed-off-by: Michael Brandl <git@fineon.pw>