clocksource: sirf/marco+prima2: drop usage of CLOCK_TICK_RATE

Since CSR SiRF was converted to multi platform in cf82e0e4 (ARM: sirf:
enable multiplatform support) the symbol CLOCK_TICK_RATE isn't the
platform specific definition any more, but a global dummy value. There
was no harm introduced in cf82e0e4

 because the global value happens to
match the old platform specific one, still this dummy value isn't
intended to be used and will hopefully disappear soon, so introduce a
local #define and use that instead.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>