ci: add yocto-check-layers step

Add a job to run yocto-check-layers to validate a selection of layers
are Yocto Project Compatible.

Right now we validate:
- meta-arm
- meta-arm-bsp
- meta-arm-toolchain
- meta-gem5

meta-atp and meta-arm-autonomy are currently skipped: meta-atp fails and
meta-arm-autonomy has a larger set of dependant layers and will be added
later.

Change-Id: Ia0c8c16e4228eeb461bd213b30703e950ede656f
Signed-off-by: Ross Burton <ross.burton@arm.com>