DS1307: Add the mcp7941x enable bit

Summary:
Existing code recognizes the mcp7941x RTC, but this RTC has an
enable bit at the same location as the "Clock Halt" bit on the ds1307, with an
opposite assertion (set == on, whereas CH set == clock stopped). Thus the
current code halts the clock, with no way to enable it.

Reviewed By: ian
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12961