docs(gpt): update GPT library documentation

This patch updates GPT library design documentation
with the changes introduced by patches which add
support for large GPT mappings and configuration of
memory size protected by bitlock.

Change-Id: I1f97fa8f003deb07a5f32b7237c1927581a788c8
Signed-off-by: AlexeiFedorov <Alexei.Fedorov@arm.com>
1 file changed