dt-bindings: phy: cadence-torrent: update reference file of docs

In commit fd7abc3c

 ("phy: cadence-torrent: Use a common header
file for Cadence SERDES"), phy-cadence-torrent.h was renamed to
phy-cadence.h. Fix it of the Documentation.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20210506114940.22215-1-wanjiabing@vivo.com


Signed-off-by: Rob Herring <robh@kernel.org>