dt-bindings: phy: Document WIZ (SERDES wrapper) bindings

Add DT binding documentation for WIZ (SERDES wrapper). WIZ is *NOT* a
PHY but a wrapper used to configure some of the input signals to the
SERDES. It is used with both Sierra(16G) and Torrent(10G) serdes.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[jsarha@ti.com: Add separate compatible for Sierra(16G) and Torrent(10G)
 SERDES]
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>