clocksource/drivers/pxa_timer: Move the Kconfig rule

Instead of having the clocksource's Kconfig depending on the arch, let the
arch to select the timer it needs.

The CLKSRC_OF dependency is removed because already selected by the
ARCH_PXA, and it is added for SA1100.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>