UPSTREAM: net: phy: dp83867: Add documentation for optional impedance control

UPSTREAM: net: phy: dp83867: Add documentation for optional impedance control

Add documention of ti,min-output-impedance and ti,max-output-impedance
which can be used to correct MAC impedance mismatch using phy extended
registers.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit d6081de7e011327af089475bb60593423963526a)
Change-Id: I5e1f90caff7fee13369302a84d1dac370cb75f5e
Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>