main/linux-purism-librem5: upgrade to 5.16.18

several notable changes:

1. broadcom and marvel drivers enabled, and brcm firmware package added -- these modules can be installed in the L5, I have the brcm one now which is why I also included the fw package. enabling marvel just makes our kconfig match purism's a little more closely

2. modem PM re-enabled -- big impact on battery life, but the modem can still disappear completely... there's a patch to make it happen much less frequently, which I feel is a good compromise for now.