atkbdc(4): Add quirk for "System76 lemur Pro" laptops.

Currently atkbdc(4) assumes all coreboot BIOSes belonging to Chromebooks
and unconditionally sets a number of quirks to workaround known issues.

Exclude "System76" laptops from this set as they appeared to be a
traditional hardware ("lemur Pro" is a rebranded Clevo chassis) with
coreboot firmware on board. KBDC_QUIRK_KEEP_ACTIVATED quirk activated for
Chromebook platform makes keyboard on this devices inoperable.

"Purism Librem" laptops may require the same exclusion too.

PR: 250711
Reported by: nick.lott@gmail.com
MFC after: 2 weeks