- Remove `mret` for jumping to the service call dispatcher; instead, enable
interrupts and execute directly
- Fix potential corruption of the `t3` register when returning from a service
call
- Simplify the secure service dispatcher function
Also:
- Split the secure service table into two parts: one DRAM-resident and the
other DROM-resident. The former holds the services invoked when the cache is
disabled or suspended while the latter holds rest of the services.