Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFp4: Add the ipend() method to the serdev I/F to allow umbrella
drivers to obtain pending interrupt status from subordinate
drivers.

Add scc(4), a driver for serial communications controllers. These
controllers typically have multiple channels and support a number
of serial communications protocols. The scc(4) driver is itself
an umbrella driver that delegates the control over each channel
and mode to a subordinate driver (like uart(4)).
The scc(4) driver supports the Siemens SAB 82532 and the Zilog
Z8530 and replaces puc(4) for these devices.