It turns out hooking up an unused peripheral to a disabled GCLK
leads to surprising power savings.
Name the GCLK to be more explicit (and since not all members of
the extended samd2x family have a GCLK7).
Turns out we can just use a non-existing GCLK ID for this, this
even saves us a real GCLK that we can use for something else.
Also make sure to disable *all* peripherals by using
`GCLK_CLKCTRL_ID_Msk` instead of relying on a magic value.
Looks like we previously missed some, since this leads to some
additional power savings:
master: 4.22 mA
this patch: 4.09 mA
91604d0217