usb: dwc3-meson-g12a: Select PHY instead of imply PHY

Imply means you can turn off the option and expect things to work
- "it's a good idea to have X enabled" is when to use imply
- "you must have X for Y to work" is when to use select

Use "select" here.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>