mmc: Don't suggest to build modules in Kconfig.

U-Boot does not support building kernel modules.

Fixes: 3c0dbed2 ("mmc: arm_pl180_mmci: adapt driver to DM usage")
Fixes: 36645f45 ("drivers: mmc: Add sdhci driver for Broadcom iProc platform")
Fixes: dadd43c1 ("mmc: synquacer: Add SynQuacer F_SDH30 SDHCI driver")
Fixes: b312c590 ("mmc: Add MMC support for stm32h7 Socs")
Fixes: d24b6939 ("mmc: mtk-sd: add SD/MMC host controller driver for MT7623 SoC")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>