ARM: SP804: make Integrator/CP timer pick clock from DT

This modifies the SP804 driver so that the clock will be taken
from the device tree node for the timer.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Rob Herring <rob.herring@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>