dt-bindings: net: dwmac: Fix typo in the R40 compatible

Even though both the driver and the device trees all use the
allwinner,sun8i-r40-gmac compatible, we documented the compatible as
allwinner,sun8i-r40-emac in the binding. Let's fix this.

Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jose Abreu <joabreu@synopsys.com>
Cc: netdev@vger.kernel.org
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210924072756.869731-3-maxime@cerno.tech