staging: fsl-dpaa2/rtc: support phc_index of ethtool_ts_info

This patch is to support phc_index of ethtool_ts_info.
Also make the rtc drvier depend on FSL_DPAA2_ETH because
this driver is only useful when PTP programs are getting
hardware time stamps on the PTP Ethernet packets using the
SO_TIMESTAMPING API.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>