samples: drivers: rtc: change hard-coded device nam

Change to use the device config name defined by driver's Kconfig
for device binding, instead of hard-coding it which is not
portable.

Jira: ZEP-1764

Change-Id: I7af234ada73302eb062340740df2fc7a8539150d
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>