net: phy: dp83867: Add ability to disable output clock

Based on commit 13c83cf8af0d ("net: phy: dp83867: Add ability to disable
output clock") of mainline linux kernel.

Generally, the output clock pin is only used for testing and only serves as
a source of RF noise after this.  It could be used to daisy-chain PHYs, but
this is uncommon.  Since the PHY can disable the output, make doing so an
option.  I do this by adding another enumeration to the allowed values of
ti,clk-output-sel.

The code was not using the value DP83867_CLK_O_SEL_REF_CLK as one might
expect: to select the REF_CLK as the output.  Rather it meant "keep clock
output setting as is", which, depending on PHY strapping, might not be
outputting REF_CLK.

Change this so DP83867_CLK_O_SEL_REF_CLK means enable REF_CLK output.
Omitting the property will leave the setting as is (which was the previous
behavior in this case).

Out of range values were silently converted into DP83867_CLK_O_SEL_REF_CLK.
Change this so they generate an error.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>