net: include/phy.h: add new mode for internal phy

Add the new mode to indicate a built-in PHY.
This will be used by UniPhier AVE ethernet driver.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>