usb: dwc3-meson-g12a: add power-on/off of the PHYs

Power on/off the PHYs to enable power to the USB ports, fixing USB support
on Khadas VIM3/VIM3L boards.

The G12A USB complex has at least 2 USB2 PHYs, but one is muxed between the
DWC2 and DWC3 controller and the other one directly connected to the DWC3
controller. The USB3+PCIe combo PHY is muxed between the DWC3 controller
and a DW-PCIE controller.
All PHYs are optional, but it's type (usb2/usb3) and position are important
to determine it's capabilities, thus they are stored in a fixed size
array and the phy-name determines it's position, it's position determining
it's type and functionnalities.
This is why we need to loop over the array to power on all the DT provided
PHYs.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Marek Vasut <marex@denx.de>