dt-bindings: memory: mt7621: add syscon as compatible string

The syscon string was introduced because the mt7621 clock driver needs to
read some registers creating a regmap from the syscon. The bindings were
added before the clock driver was properly mainlined and at first the clock
driver was using ralink architecture dependent operations rt_memc_* defined
in 'arch/mips/include/asm/mach-ralink/ralink_regs.h'.

This string is already there on the memory controller node on mt7621.dtsi.

Add syscon as a constant string on the compatible property, now that memc
became a syscon. Update the example accordingly.

Fixes: 5278e4a181ff ("dt-bindings: memory: add binding for Mediatek's MT7621 SDRAM memory controller")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/memory-controllers/mediatek,mt7621-memc.yaml

@@ -11,7 +11,9 @@ maintainers:
 
 properties:
   compatible:
-    const: mediatek,mt7621-memc
+    items:
+      - const: mediatek,mt7621-memc
+      - const: syscon
 
   reg:
     maxItems: 1
@@ -25,6 +27,6 @@ additionalProperties: false
 examples:
   - |
     memory-controller@5000 {
-        compatible = "mediatek,mt7621-memc";
+        compatible = "mediatek,mt7621-memc", "syscon";
         reg = <0x5000 0x1000>;
     };