Add the ability to modify clusters

[jessebraham]

In a vendor-supplied SVD file, the following layout was encountered within a peripheral:

<registers>
  <register>...</register>
  <register>...</register>
  <cluster>
    <name>CHANNEL[%s]</name>
    <description />
    <register>...</register>
    <register>...</register>
  </cluster>
</registers>

The ability to modify the <cluster> tag's description attribute was needed, however this did not appear to be possible. This PR is just a quick and dirty method of accomplishing such. Patching can be performed by:

DMAC:
  _modify:
    _cluster:
      "CHANNEL*":
        description: "Direct Memory Access Channels"

I've sort of repurposed _cluster here, but this shouldn't cause any problems (as far as I can tell) as it's being used in a new context. If there are any concerns with this approach, or any other changes need to be made, please just let me know and I'll be happy to do so.

[adamgreig]

LGTM, thanks!

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push