ARM: dts: stm32: Enable MAC TX clock gating during TX low-power mode on stm32mp15

When there is no activity on ethernet phy link, the ETH_GTX_CLK is cut.

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>