superio/nuvoton/nct5539d: Update documentation and remove DSDT

There seems to be no board using this, but some currently under review.
Remove the DSDT, which doesn't work together with the SSDT ACPI
code generation. Also update the documentation pointing to the SSDT
generator.

Change-Id: I8b7daeadaaac93d74ee2fc9eb18f0eff5ef50eb3
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38864
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/superio/index.md

@@ -5,6 +5,7 @@ This section contains documentation about coreboot on specific SuperIOs.
 ## Nuvoton
 
 - [NPCD378](nuvoton/npcd378.md)
+- [NCT5539D](nuvoton/nct5539d.md)
 
 ## Common
 - [PNP devices](common/pnp.md)

Documentation/superio/nuvoton/nct5539d.md

@@ -0,0 +1,9 @@
+# NCT5539D SuperIO
+
+The SuperIO has the ID `0xd121` and the source can be found in
+`src/superio/nuvoton/nct5539d/`.
+
+## For developers
+
+The SuperIO generates ACPI using the
+[SSDT generator for generic SuperIOs](../common/ssdt.md).