dts: add 'linaro,' prefix to 96b-lscon-1v8, 96b-lscon-3v3

These compatibles don't match the dt-schema regular expression for
compatible properties that we'd like to support in Zephyr because they
do not begin with a letter.

Use linaro, as a vendor prefix to make them compliant. Update the
release notes since out of tree users will need to keep up.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>