MAINTAINERS: Use my dedicated work GitHub account

While GitHub encourages using single account for both personal and work
purposes (even recommends merging multiple accounts into one), there are
some serious downsides when doing so.

The biggest disadvantage of using one account is the inability to select
which email address the "Rebase and merge" action will use. GitHub, at
least as of today, always uses primary email address for "Rebase and
merge" action.

Another issue is selecting email for notification based on organization.
Custom email routing can only be configured if you are organization
member (or at least outside collaborator). While this works reasonably
fine for zephyrproject-rtos organization, I cannot route e.g. mcu-tools
organization emails to my work email.

To avoid the issues I have decided to use dedicated GitHub account for
my Nordic Semiconductor related work activities: tmon-nordic

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>