Reduce the number of header files included from other header
files as much as possible without splitting the files. Use forward
declarations where possible. This allows removal of some unnecessary
"#ifndef __ASSEMBLY__" statements.
Also, review the .c and .S files for which header files really need
including and reorder the #include statements alphabetically.
FixesARM-software/tf-issues#31
Change-Id: Iec92fb976334c77453e010b60bcf56f3be72bd3e