[topgen] Only create pwrmgr/pinmux/alert_handler if there is an instance

[Razer6]

Not all tops may have a pwrmgr, pinmux, alert_handler or clkmgr. Only create those IPs and the related connections if the top really specifies those IPs.

[andreaskurth]

LGTM in the sense that it doesn't negatively impact currently generated tops. Whether the result is correct for tops that make use of the added functionality I cannot say, though

[Razer6]

This check was broken as it only checked the rstmgr but not the others due to operator precedence.

[Razer6]

I used it downstream, and it works. The used top-level template though needs some tweaks, but I leave that as a downstream issue.

One thing I needed to change was the order of the alert and plic mappings in toplevel.c.tpl. The alert mappings were at the end of that file, and gating it with an if caused mako to render two newlines. One for the mapping and one for the endif. This then caused a linting issue. The easy fix was to put the un-gated plic mapping at the end.

I had to add a special case for Englishbreakfast. Although this top does not have an alert handler in the design it uses SW from it. It copies its own alert handler config over to Earlgrey and then build SW from that. Here, In this PR we ensure that the alert handler is generated for Englishbreakfast even though the design does not have an actual alert handler.