rtc: xgene: Set hardware dependency

The rtc-xgene driver is only useful on X-Gene SoC.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Iyappan Subramanian <isubramanian@apm.com>
Cc: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>