Announce flow control ability to PHY driver and enable RX flow

control.  Controller does not automatically generate pause frames
based on number of available RX buffers so it's very hard to
know when driver should generate XON frame in time.  The only
mechanism driver can detect low number of RX buffer condition is
ET_INTR_RXRING0_LOW or ET_INTR_RXRING1_LOW interrupt.  This
interrupt is generated whenever controller notices the number of
available RX buffers are lower than pre-programmed value(
ET_RX_RING0_MINCNT and ET_RX_RING1_MINCNT register).  This scheme
does not provide a way to detect when controller sees enough number
of RX buffers again such that efficient generation of XON/XOFF
frame is not easy.

While here, add more flow control related register definition.

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