cmd: date: rtc: Update command to read the first RTC with seq 0

cmd: date: rtc: Update command to read the first RTC with seq 0

RTCs are using sequence number defined in aliases node. Date command with
DM_RTC enabled is looking for the first RTC with index 0. But when
RTC_EMULATION is enabled it gets likely most of the time index 0 even when
system has rtc0 device via aliases node and gets sequence number 0.
That's why extend the code to look for sequence 0 number first. If this
fails continue to use existing device with index 0.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>