In am335x_dmtpps, use a spin mutex to interlock between PPS capture and PPS
ioctl(2) handling. This allows doing the pps_event() work in the polling
routine, instead of using a taskqueue task to do that work.

Also, add PNPINFO, and switch to using make_dev_s() to create the cdev.

Using a spin mutex and calling pps_event() from the polling function works
around the situation which requires more than 2 sets of timecounter
timehands in a single-core system to get reliable PPS capture. That problem
would happen when a single-core system is idle in cpu_idle() then gets woken
up with an event timer event which was scheduled to handle a hardclock tick.
That processing path would end up calling tc_windup 3 or 4 times between
when the tc polling function was called and when the taskqueue task would
eventually run, and with only two sets of timehands, the th_generation count
would always be too old to allow the captured PPS data to be used.