ARM: dts: am57xx-beagle-x15: Fix RTC aliases

ARM: dts: am57xx-beagle-x15: Fix RTC aliases

With commit bc078316d86c ("ARM: dts: DRA7: Add node for RTC"), we now
have AM57xx RTC register itself as alias 0 even before DS1307 or TPS
rtc drivers are loaded up. However, since neither TPS, nor AM57xx RTC
are capable of being backedup by battery, we would like to maintain
the "primary" rtc as mcp79410 rtc device.

This also generates the following warnings in the bootlog highlighting
the issue:
[    5.895445] rtc-ds1307 2-006f: /aliases ID 0 not available
...
[    6.476285] palmas-rtc 48070000.i2c:tps659038@58:tps659038_rtc: /aliases ID 1 not available

So, add proper aliases to ensure that RTC order is always consistent
to userspace immaterial of probe order.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>