xtensa: standardize devicetree cpu compatible strings

The recommended compatible string format, according to the ePAPR v1.1 standard,
is "manufacturer,model". Change the xtensa cpu compatible strings to
"cdns,xtensa-cpu". Also, change the boards compatible strings in a similar way.

The pic compatible string will be dealt with in a separate patch.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>