mmc: dt: Consolidate DT bindings

This patch unifies the current DT MMC bindings documentation and code,
adds generic MMC DT bindings documentation, and updates .dts files for
consistency.

[cjb: typo fixes, addition of max-frequency property]
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/powerpc/boot/dts/mpc8569mds.dts

@@ -119,6 +119,7 @@
 		sdhc@2e000 {
 			status = "disabled";
 			sdhci,1-bit-only;
+			bus-width = <1>;
 		};
 
 		par_io@e0100 {