ARM: dts: doc: Document missing compatible property for omap-sham driver

A new compatible property "ti,omap5-sham" is added to the omap-sham driver
recently to support SHA/MD5 for OMAP5,DRA7 and AM43XX. Documenting the
same.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>