phy: Add Amlogic Meson USB2 & USB3 Generic PHY drivers

The Amlogic Meson GXL and GXM (simple variant) embeds up to 3 USB2 PHYs
and an USB3 PHY. This patch adds drivers for these for the standard generic
PHY interface and supports the power-on/off calls and set the Host mode by
default.
They are based on the excellent work from Martin Blumenstingl merged in linux.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>