rtc: honor device tree /alias entries when assigning IDs

Assign RTC device IDs based on device tree /aliases entries if present,
falling back to the existing numbering scheme if there is no /aliases
entry (which includes when the system isn't booted using DT), or there is
a numbering conflict.

This is useful in systems with multiple RTC devices, to ensure that the
best RTC device is selected as /dev/rtc0, which provides the overall
system time.

For example, Tegra has an on-SoC RTC that is not battery backed, typically
coupled with an off-SoC RTC that is battery backed.  Only the latter is
useful for populating the system time, yet the former is useful e.g.  for
wakeup timing, since the time is not lost when the system is sleeps.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>