docs: networking: device drivers: convert cirrus/cs89x0.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/index.rst

@@ -32,6 +32,7 @@ Contents:
    amazon/ena
    aquantia/atlantic
    chelsio/cxgb
+   cirrus/cs89x0
 
 .. only::  subproject and html
 

drivers/net/ethernet/cirrus/Kconfig

@@ -24,7 +24,7 @@ config CS89x0
 	---help---
 	  Support for CS89x0 chipset based Ethernet cards. If you have a
 	  network (Ethernet) card of this type, say Y and read the file
-	  <file:Documentation/networking/device_drivers/cirrus/cs89x0.txt>.
+	  <file:Documentation/networking/device_drivers/cirrus/cs89x0.rst>.
 
 	  To compile this driver as a module, choose M here. The module
 	  will be called cs89x0.