padlock: Restore padlock_rng to the amd64 build

[asomers]

Summary

Revert 88a53301e19 and d1ca01059d5 . They removed padlock_rng from the
amd64 build under the mistaken belief that this device was available on
32-bit processors only. But it's also available on the 64-bit Via Nano,
Nano X2, and some Zhaoxin CPUs.

PR: 295517
Fixes: 88a53301e19 ("padlock.4: only install on i386")
Fixes: d1ca01059d5 ("padlock(4)/nehemiah: move i386-only entropy")
MFC after: 1 week

Test Plan

Loaded the driver on a Via Nano X2 system and checked kern.random