schemas: legacy: add MP5998 device

Add MP5998 to PSUSensor types enum.

Change-Id: I3a7c5a00a3b49a958707fd7d3e9ba8ac8e450191
Signed-off-by: Leo Yang <Leo-Yang@quantatw.com>
diff --git a/schemas/legacy.json b/schemas/legacy.json
index e24363d..eda69ea 100644
--- a/schemas/legacy.json
+++ b/schemas/legacy.json
@@ -945,6 +945,7 @@
                         "MP2975",
                         "MP5023",
                         "MP5990",
+                        "MP5998",
                         "MP29612",
                         "MPQ8785",
                         "NCP4200",