staging: greybus: add maintainer for greybus platform drivers

Add Vaibhav Hiremath as a Maintainer of Greybus platform
drivers.

Signed-off-by: Vaibhav Hiremath <hvaibhav.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>