clocksource/drivers/u300: Add the COMPILE_TEST option

Change the Kconfig option logic to fullfil with the current approach.

A new Kconfig option is added, CONFIG_U300_TIMER and is selected by the
platform. Then the clocksource's Kconfig is changed to make this option
selectable by the user if the COMPILE_TEST option is set. Otherwise, it is
up to the platform's Kconfig to select the timer.

Due on the delay specific code, this driver will compile only on the ARM
architecture.

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