clocksource/drivers/tcb_clksrc: Make IO endian agnostic

Now that AVR32 is gone, we can use the proper IO accessors that are
correctly handling endianness.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>