doc: Update list of maintainers for SCP-firmware
This commit updates the maintainers documentation to reflect changes to the project maintainership and review process.
It also adds a CODEOWNERS file, the format of which is defined by GitHub, describing the same information.
Change-Id: Ibe631731b633eb0389efe1f122d8ca4308216926 Co-authored-by: Jim Quigley jim.quigley@arm.com Signed-off-by: Chris Kay chris.kay@arm.com Signed-off-by: Jim Quigley jim.quigley@arm.com