doc: Update communication tools

Prefer to use the trustedfirmware.org collaboration tools (i.e. issues
tracker and development mailing list) over the #trusted-firmware-a IRC
channel.

Change-Id: I78aef022b00cd553e1ab6542b967dd3f0640dd52
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>