armv8r64-extras/zephyr: Switch to v3.0-branch release branch

The Zephyr 3.0.x release branch now contains all the backports, so
switch to that branch and remove the downstream patches.

Issue-Id: SCM-4304
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Icedc6d814269cb0e731d8464086df26b6242b7b5