TZC: rename included C file to a header

TZC: rename included C file to a header

C files shouldn't be included into others. This file only contains some
macros and functions that can be made `static inline`, so it is ok to
convert it into a header file.

This is the only occurrence of a C file being included in another one in
the codebase instead of using a header, other occurrences are a way of
achieving backwards-compatibility.

Functions therein have been qualified as `inline`.

Change-Id: I88fe300f6d85a7f0740ef14c9cb8fa54849218e6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>