xtensa: support common clock framework

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: xtfpga: use common clock framework

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: xtfpga: set ethoc clock frequency

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>