drivers/ipmi: Add chip ops

* Add chips ops for IPMI KCS.
* Get IPMI version over KCS.
* Generates ACPI SPMI table for IPMI KCS.
* Generates SMBIOS type 38 for IPMI KCS.
* Generates ACPI SPMI device for IPMI KCS on LPC device.
* Add documentation

To use this driver on BMC that support KCS on I/O:

1. Add an entry to the devicetree.cb:

 chip drivers/ipmi
    device pnp ca2.0 on end         # IPMI KCS
 end

2. Select IPMI_KCS in Kconfig.
3. (Optional) enable LPC I/O decode for the given address.

Tested on Wedge100s.

Change-Id: I73cbd2058ccdc5395baf244f31345a85eb0047d7
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33255
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

Documentation/drivers/index.md

@@ -0,0 +1,7 @@
+# Platform indenpendend drivers documentation
+
+The drivers can be found in `src/drivers`. They are intended for onboard
+and plugin devices, significantly reducing integration complexity and
+they allow to easily reuse existing code accross platforms.
+
+* [IPMI KCS](ipmi_kcs.md)