MAINTAINERS: Add maintainers for rmnet

Add myself and Sean as maintainers for rmnet driver.

Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>