psm(4), evdev support part 1: atkbdc access locking

[wulf_cicgroup.ru]

Summary

atkbdc(4): Serialize hardware access with mutex

In 4.x days it was serialized with spl() calls and kbdc lock flag
After 5.x it became protected with giant lock.
Restore 4.x logic with replacing kbdc lock flag acquiring with
atkbdc lock aquiring. atkbdc access is still protected with giant
too as both children are giant-locked.
spl() calls are left inplace to help psm and atkbd drivers locking.
atkbdc arbitration comments are deleted as obsoleted.

Part of https://reviews.freebsd.org/D10207

[wulf_cicgroup.ru]

Change is obsolete.
After some discussion with bde@ (on arch@ and private) it is turned out that nothing but Giant can be used to serialize access to I8042 ports currently.