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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>