Add documentation for ARM Trusted Firmware

Adds documentation how to compile and run with
ARM Trusted Firmware.
MMU: Simplify tee_mmu_switch function

Replaced direct CP15 instructions with macros instead and removed some
unnecessary code.
Make license terms more explicit

It fixes #37 "README.md is unclear about license"
Makefile variables $(*-dir) should not have a trailing slash

As a general rule, Makefile variables that are directories should not have
a trailing slash, and should be used as: $(some-dir)/some-file rather than
$(some-dir)some-file. This is more readable.
Generic secure time layer

Provided a time source api in order to let soc vendors implement their
own secure counter

- Moved platform-dependent code out of tee_time.c to make it a generic
  time layer.
- Added an abstract layer for platforms to implement their own secure
  time source.
- Implemented arm cntpct as one of secure time source.
- Moved rtt0 related time operation from tee_time.c to tee_time_rtt.c,
  act as another secure time source.
- Added tee_time_ree.c for the plaform that doesn't have secure time
  source. In this case, using ree time as secure time source.

minor change as per comments