Documentation: ARM: List new omap MMC requirements

Earlier the PBIAS regulator was optional, not so with recent
omap_hsmmc changes. To make things easier for people with
custom .config files, let's add minimal documentation for it
as suggested by Russell King <rmk+kernel@arm.linux.org.uk>.
Signed-off-by: Tony Lindgren <tony@atomide.com>

Documentation/arm/OMAP/README

+This file contains documentation for running mainline
+kernel on omaps.
+
+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.