xtensa: support common clock framework

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

With this change the board needs to set up single clock object, users of
this clock will get correct frequency automatically.

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

Connect xtfpga board ethernet MAC to the clock in the DTS. Set up MAC
base frequency in the platform data in case of build w/o CONFIG_OF.

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