[top_earlgrey,pinmux] Default pull down for IOC8 (TAP strap 0)

Earlgrey's IOC8 pad is used as TAP strap 0. [TAP strap 0 gets sampled at least once at boot in every LC state.](https://opentitan.org/book/hw/ip/pinmux/doc/theory_of_operation.html#strap-sampling-and-tap-isolation) Thus if the pad is not properly pulled down on a PCB and gets sampled as 1 at boot, the pads used for JTAG are not available as MIOs until the next reset (even if the LC state does not allow JTAG debug). To prevent this, we need to activate the pad-internal pull down of IOC8 at reset.

~~As the pad attributes of IOC8 are part of a multireg, we currently cannot set their reset value separately. We probably have to extend our multireg syntax and features to allow this.~~

This change needs to be applied on the `earlgrey_1.0.0` release branch (an ECO confirmed by a local RTL modification has already happened) as well as on `master`.