MFC r330773, r330778, r330782, r330797

r330773:
Use separate mutexes for atrtc and i8254 locking. Change all the strange
un-function-like RTC_LOCK/UNLOCK macro usage into normal function calls.
Since there is no longer any need to handle register access from a debugger
context, those function calls can just be regular mutex lock/unlock calls.

Requested by: bde

r330778:
Everywhere that multiple registers are accessed in sequence, lock/unlock
just once around the whole group of accesses.

r330782:
Remove MTX_NOPROFILE from atrtc_lock, it was inappropriately copy/pasted
from the i8254 driver when I created separate mutexes for each. The i8254
driver could be the active timecounter, leading to recursion during mutex
profiling, but the atrtc driver cannot be a timecounter, so it isn't needed.

r330797:
Give the atrtc_time_lock a unique name.

Reported by: hps@