[WATCHDOG] davinci: use clock framework for timer frequency

Remove use of CLOCK_TICK_RATE in favor of using clock framework
for getting timer frequency.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>