cc2538: sys_ctrl: Make it possible to use the 32-kHz crystal

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge branch 'cc2538-sys_ctrl' into qidevices

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>