[WATCHDOG] Basic support for GE Fanuc's FPGA based watchdog timer

GE Fanuc SBC610

Support for the FPGA based watchdog timer as found on GE Fanuc's SBC310,
SBC610 and PPC9A Single Board Computers.

This patch adds support for the watchdog timer found in one of the devices
FPGAs. There are two identical watchdog timers at different offsets in the
above mentioned boards, this driver is capable of supporting one of them.
The watchdog timers are also capable of generating interrupts at a
user-configurable threshold, though support for this operation is currently
not supported by the driver.

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>