cassini-[doc,config,distro]: Cassini 0.9.0 release

For the v0.9.0 release,:

• Sets DISTRO_VERSION to v0.9.0
• Pins all dependent layers
• Updates the documentation mentioning the v0.9.0 in the relevant sections

Changelog: other

Signed-off-by: Devaraj Ranganna devaraj.ranganna@arm.com