dt-bindings: phy: cadence-torrent: Add binding for refclk driver

Add binding for refclk driver used to route the refclk out of torrent
SERDES.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210310112745.3445-3-kishon@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>