hmt(4): Add support for serial packet reporting mode

In Serial mode, each packet contains information that describes a
single physical contact point. Multiple contacts are streamed
serially. In this mode, devices report all contact information in a
series of packets. The device sends a separate packet for each
concurrent contact.

Serial packet reporting mode is used by post-2014 eGalax touchscreens.

Tested by: Mark Kane <mark_AT_kane_DOT_mn>
MFC after: 2 weeks