net: phy: dp83867: implement support for io_impedance_ctrl nvmem cell

We have a board where measurements indicate that the current three
options - leaving IO_IMPEDANCE_CTRL at the (factory calibrated) reset
value or using one of the two boolean properties to set it to the
min/max value - are too coarse.

Implement support for the newly added binding allowing device tree to
specify an nvmem cell containing an appropriate value for this
specific board.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>