Update config for v1.0 with kernel v5.15

This configuration allows building an ACS-IR image close to v1.0, but with
a Linux kernel updated to v5.15.

This is of interest for systems supported only by a recent kernel.

Also, add a warning in the README.

Suggested-by: G Edhaya Chandran <edhaya.chandran@arm.com>
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>