google/chromeec: Add support for "base attached switch" device

On some detachables, the mere presence of attached base is not enough to
determine whether the device is in tablet mode or not, so we introducing
a new "switch" in EC, separate from "Tablet Mode" switch, to signal
whether the base is attached or not.

We also want the driver to be separate from cros_ec_keyb, so we create
a new ACPI device, C(hrome)B(ase)A(ttached)S(witch), with HID GOOG000B,
and guard it with EC_ENABLE_CBAS_DEVICE.

Change-Id: Id73a12f04a1a48f7fbd9365c2a501afadf3878fa
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Reviewed-on: https://review.coreboot.org/28260
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/ec/google/chromeec/acpi/cros_ec.asl

@@ -30,4 +30,13 @@ Device (CREC)
 		Name (_DDN, "EC MKBP Device")
 	}
 #endif
+
+#ifdef EC_ENABLE_CBAS_DEVICE
+	Device (CBAS)
+	{
+		Name (_HID, "GOOG000B")
+		Name (_UID, 1)
+		Name (_DDN, "EC Base Switch Device")
+	}
+#endif
 }