bindings: net: stmmac: correctify note about LPI interrupt

There are two different combined signal for various interrupt events:
In EQOS-CORE and EQOS-MTL configurations, mci_intr_o is the interrupt
signal.
In EQOS-DMA, EQOS-AHB and EQOS-AXI configurations, these interrupt events
are combined with the events in the DMA on the sbd_intr_o signal.

Depending on configuration, the device tree irq "macirq" will refer to
either mci_intr_o or sbd_intr_o.

The databook states:
"The MAC generates the LPI interrupt when the Tx or Rx side enters or exits
the LPI state. The interrupt mci_intr_o (sbd_intr_o in certain
configurations) is asserted when the LPI interrupt status is set.

When the MAC exits the Rx LPI state, then in addition to the mci_intr_o
(sbd_intr_o in certain configurations), the sideband signal lpi_intr_o is
asserted.

If you do not want to gate-off the application clock during the Rx LPI
state, you can leave the lpi_intr_o signal unconnected and use the
mci_intr_o (sbd_intr_o in certain configurations) signal to detect Rx LPI
exit."

Since the "macirq" is always raised when Tx or Rx enters/exits the LPI
state, "eth_lpi" must therefore refer to lpi_intr_o, which is only raised
when Rx exits the LPI state. Update the DT binding description to reflect
reality.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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