rtc: pxa: drop unused #define TIMER_FREQ

It seems this symbol was unused since the driver was introduced in
commit dc944368 (rtc: driver for pxa27x and pxa3xx SoC) back in 2009.

As a by-product this patch makes the driver stop "using" the symbol
CLOCK_TICK_RATE which is about to be removed very soon (for ARM).

Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>