MAINTAINERS: Update entry for ibmvmc driver

Steve Royer has moved on to a different project and has asked
that Ritu and I take over maintainership of the IBM Power
Virtual Management Channel Driver.
Signed-off-by: Brad Warrum <bwarrum@linux.ibm.com>
Acked-by: Steven Royer <seroyer@linux.ibm.com>
Link: https://lore.kernel.org/r/20210330212238.2747-1-bwarrum@linux.ibm.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -8552,7 +8552,8 @@ S:	Supported
 F:	drivers/scsi/ibmvscsi/ibmvfc*
 
 IBM Power Virtual Management Channel Driver
-M:	Steven Royer <seroyer@linux.ibm.com>
+M:	Brad Warrum <bwarrum@linux.ibm.com>
+M:	Ritu Agarwal <rituagar@linux.ibm.com>
 S:	Supported
 F:	drivers/misc/ibmvmc.*