A better HintHandler

This reverts commit 9a30c4ba.

debug: fix RESUMING desc

Establish mimpid scheme; set to initial value

Every time we bless a release of rocket-chip, we'll set mimpid to
the current date as a BCD string, e.g., 0x20181004 for today.

Correct one of the reg field descriptions for RESUMING address

As AHB does not support PutPartial and often connects to devices
which have side effects on Gets, the HintHandler cannot provide
Hint support for AHB slaves. Therefore, support Hints directly.

More Rocket clock gating

This finishes the work started in 1d95fcc8

...but not during WFI.

This reverts commit 8d4aa828, reversing
changes made to b688dc71.

This reverts commit c9f5f417, reversing
changes made to 1d95fcc8.

Better HintHandler

Refactor DevNullDevices and provide some within TLBusWrappers

Add Simple Custom Instruction Extension

This enables adding custom instructions that operate on the x-registers
(i.e., no additional architectural state).

This version on supports combinational units (which fit in the pipeline in
the same place as the integer ALU), but it can be extended to support 2-stage
pipelines, or multi-cycle operations.

Adding a no-argument fire() to DecoupledHelper that doesn't exclude anything. The previous way of doing this, which was fire(false.B), was broken by the previous change of requiring all excluded to be included, and forced a further hack of adding a dummy value. Since excluding false.B was a hack in itself, this avoids the issue entirely by providing a straightforward and safe way of excluding nothing.