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

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

Add convenience macros for the full register set and use them to replace
magic constants in clkbrd.c

Info from: OpenSolaris

Add a driver for the 'clock-board' device (the clock board is an
inevitable component in Sun Exx00 machines and provides serial ports,
NVRAM and TOD amongst others which are handled by uart(4) and eeprom(4)
respectively). This driver currently only prints out information about
the chassis on attach and allows to blink the 'Cycling' LED (which is
duplicated on the front panel) of the clock board just like fhc(4) does
for the other boards. The device name for the LED is /dev/led/clockboard.

Obtained from: OpenBSD
Tested by: joerg