watchdog: sirf: don't depend on dummy value of CLOCK_TICK_RATE

As CSR SiRF is converted to multi platform CLOCK_TICK_RATE is a dummy
value that seems to match the right value is used.
(arch/arm/mach-prima2/include/mach/timex.h which defined CLOCK_TICK_RATE
to 1000000 was removed in commit cf82e0e4 (ARM: sirf: enable
multiplatform support); marco used the same file.)

To not depend on that dummy value use a local #define instead.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>