The Intel Lightning Mountain (LGM) USB3 USB is only present on Intel
Lightning Mountain SoCs.  Hence add a dependency on X86, to prevent
asking the user about this driver when configuring a kernel without
Intel Lightning Mountain platform support.

Fixes: 1cce8f73 ("phy: Add USB3 PHY support for Intel LGM SoC")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210129131753.2656306-1-geert+renesas@glider.beSigned-off-by: Vinod Koul <vkoul@kernel.org>