mtkmodem: Add defines for all MTK specific events

Add defines and change request names to show that they are specific to
MTK.
gril: Delete marked callbacks

Callbacks were not being removed if unregistration was happening inside
a callback call, they were just being marked. This change makes sure
that marked callbacks are deleted after handling the event that
started the callback that marked them.
mtk: Handle SIM hot swap

This change re-starts atoms after detection of SIM insertion/removal
for MTK modem.
unit: Adapt tests to MTK events name change