ARM: dts: imx6qdl-hummingboard: Add rtc0 and rtc1 aliases to fix hctosys

HummingBoard has two RTCs, first integrated within SoC that can be used to
wake up from sleep - and a second on the carrier board including back-up
battery which is intended for keeping time during power-off.

Add aliases for both, ensuring that the battery-backed clock is primary
rtc and used by default during boot for restoring system time.

Fixes keeping time across power-cycle observed on Debian,
which sets RTC_HCTOSYS_DEVICE="rtc0".

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>