watchdog: sp804: enable sp804 watchdog to syscon reset register

There are 8 sp804 timers in axm56xx. Timer 5 and 6 are used to be watchdog
since their interrupt connect to System Reset or Chip Reset.

At first we modify the dts file to assign correct reg address to these
2 watchdogs according to Reference Manual.

And then we should use of_device_is_compatible to filter other sp804
timer nodes which share the same amba PCellID with sp804 watchdog since
probe function is called just according to "arm,primecell-periphid" in
dts file.

And we also need enable the wd*_reset_enable bit in Reset Control Register.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>