hw/arm/Kconfig: Have FSL_IMX6UL SoC select IMX_USBPHY

Since commit 17372bd812d, the SoC used by the mcimx6ul-evk
machine requires the IMX USB PHY component.
As this component is only used by 2 machines, do not select
it by default (it will be automatically  selected when
necessary).

Fixes: 17372bd812d ("hw/arm/fsl-imx6ul: Wire up USB controllers")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251224134644.85582-2-philmd@linaro.org>