reset: hisilicon: Add media reset controller binding

Add compatible for media reset controller.

Actually, there are two reset controllers in hi6220 SoC:
The peripheral reset controller bits are part of sysctrl registers.
The media reset controller bits are part of mediactrl registers.
So for the compatible part, it should contain "syscon" for both peripheral
and media reset controller.

Signed-off-by: Xinliang Liu <[e-mail]>
Signed-off-by: Philipp Zabel <[e-mail]>

Documentation/devicetree/bindings/reset/hisilicon,hi6220-reset.txt

···
 hi6220 SoC.
 
 Required properties:
-- compatible: may be "hisilicon,hi6220-sysctrl"
+- compatible: should be one of the following:
+  - "hisilicon,hi6220-sysctrl", "syscon" : For peripheral reset controller.
+  - "hisilicon,hi6220-mediactrl", "syscon" : For media reset controller.
 - reg: should be register base and length as documented in the
   datasheet
 - #reset-cells: 1, see below