Reduce deep nesting of header files

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.

Fixes ARM-software/tf-issues#31

Change-Id: Iec92fb976334c77453e010b60bcf56f3be72bd3e
diff --git a/include/lib/semihosting.h b/include/lib/semihosting.h
index e688618..9d0b39f 100644
--- a/include/lib/semihosting.h
+++ b/include/lib/semihosting.h
@@ -31,6 +31,9 @@
 #ifndef __SEMIHOSTING_H__
 #define __SEMIHOSTING_H__
 
+#include <stdio.h> /* For ssize_t */
+
+
 #define SEMIHOSTING_SYS_OPEN            0x01
 #define SEMIHOSTING_SYS_CLOSE           0x02
 #define SEMIHOSTING_SYS_WRITE0          0x04