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

Documentation/arm/OMAP/README

@@ -5,3 +5,7 @@ KERNEL		NEW DEPENDENCIES
 v4.3+		Update is needed for custom .config files to make sure
 		CONFIG_REGULATOR_PBIAS is enabled for MMC1 to work
 		properly.
+
+v4.18+		Update is needed for custom .config files to make sure
+		CONFIG_MMC_SDHCI_OMAP is enabled for all MMC instances
+		to work in DRA7 and K2G based boards.