clocksource/drivers: Rename CLKSRC_ACPI to TIMER_ACPI

The config option name is now renamed to 'TIMER_ACPI' for consistency with
the CLOCKSOURCE_OF_DECLARE => TIMER_OF_DECLARE change.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>