Add renesas rt configs and refresh non-rt configs

Add renesas rt configs and refresh non-rt configs.

Also update top level readme.

@pavelm, these configs build okay and boot, although there is a warning seen with the renesas-rt_defconfig on the RZ/G2M platform:
https://lava.ciplatform.org/scheduler/job/13949#L669

I've tried a couple of patches that fix this warning and will get them submitted to CIP-rt soon:
https://lava.ciplatform.org/scheduler/job/14088 Kernel built from: https://gitlab.com/cip-project/cip-kernel/linux-cip/pipelines/133311132

I don't think that this warning should hold up this MR though.