MAINTAINERS: Mark subsys/mgmt/osdp as maintained

subsys/mgmt/osdp _has_ a dedicated maintainer who will rush to address
any issues identified :)

Also, add @adakus and @r2r0 as collaborators since they are using this
subsystem in production and have agreed to review, test, and report issues
for this subsystem in future.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>