dt-bindings: mfd: Document smp-memram node for AST2600 SCU

The platform initialisation code for the AST2600 implements the custom
SMP bringup protocol, and searches for the relevant compatible. As a
consequence, define the requisite node and the compatible string, which
in-turn tidies up the dtb check results.

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>