of: make of_get_phy_mode parse 'phy-connection-type'

Per the ePAPR v1.1 specification, 'phy-connection-type' is the canonical
property name for describing an Ethernet to PHY connection type. Make
sure that of_get_phy_mode() also attempts to parse that property and
update the comments mentioning 'phy-mode' to also include
'phy-connection-type'.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>