clocksource: atmel-st: select MFD_SYSCON

Enforce MFD_SYSCON selection as the driver needs it. Else, it will still
compile with some warning about X1, x2 or sr being used uninitialized.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>