phy: Add API for a bulk of phys

This patch adds a "bulk" API to the phy API in order to
get/init/exit/power on/off a group of phys associated
with a device.

The bulk API will avoid adding a copy of the same code to
manage a group of phys in drivers.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>