dt-bindings: net: dwmac-sun8i: simplify description of syscon property

The syscon property is used to point to the device that holds the glue
layer control register known as the "EMAC (or GMAC) clock register".

We do not need to explicitly list what compatible strings are needed, as
this information is readily available in the user manuals. Also the
"syscon" device type is more of an implementation detail. There are many
ways to access a register not in a device's address range, the syscon
interface being the most generic and unrestricted one.

Simplify the description so that it says what it is supposed to
describe.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>