OWNERS: update maintainer information

I will change to another assignment soon so transfer the ownership to
Thu Nguyen.
Also add Thang Tran as reviewer to review all Ampere IPMI OEM commits.

Change-Id: Idbe3e028652e080849e394a1bf7d2627fe848c5f
Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
diff --git a/OWNERS b/OWNERS
index f28ddf0..15819b0 100644
--- a/OWNERS
+++ b/OWNERS
@@ -32,17 +32,17 @@
 # data within them will be kept sorted.
 
 owners:
-- dcao-ampere
-- thangqn-ampere
+- thubanguyen
 
 reviewers:
+- thangtran
 
 matchers:
 
 openbmc:
-- name: Dung Cao
-  email: dung@os.amperecomputing.com
-  discord: dcao
-- name: Thang Q. Nguyen
-  email: thang@os.amperecomputing.com
-  discord: thangqn
+- name: Thu Nguyen
+  email: thu@os.amperecomputing.com
+  discord: thubanguyen
+- name: Thang Tran
+  email: thuutran@amperecomputing.com
+  discord: thang.tran