Documentation: ARM: Add new MMC requirements for DRA7/K2G

From 4.18 kernel, all the MMC controller instances in DRA7
are programmed using sdhci based driver (sdhci-omap.c). Document
this new requirement here. Both omap2plus_defconfig and
multi_v7_defconfig has CONFIG_MMC_SDHCI_OMAP enabled.

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