phy: intel-lgm-emmc: Add architecture dependency

The Intel eMMC PHY is only present on Intel Lightning Mountain SoCs.
Add an architecture dependency to the PHY_INTEL_EMMC config symbol, to
avoid asking the user about it when configuring a kernel for a non-x86
architecture.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200507113626.24026-3-geert+renesas@glider.be


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>