Docs: Update overall HAL design

This patch updates the overall design of HAL, including:
 - Removing loader API set. It was a long term design, currently
   it is not necessary.
 - Updating structure diagram by removing loader and debug API block.
   Loader API is being removed and debug API is yet to be worked out.

Change-Id: I07223f57ba65b6f80dcf6ee877ee9421fb4d5171
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
2 files changed