spdif: cubox-i/hummingboard: Use only PLL4 (internal) as clock source

The spdif driver will enumerate all possible clock sources (internal
and external) for a matching frequency. Since CuBox-i does not have
any external oscillator, we must tell the driver so. Otherwise it may
choose a non-present clock source resulting in no sound at 88.2 and
176.4kHz.

Signed-off-by: Rudi <r.ihle@s-t.de>