arm: dts: sync rtc node of am335x boards with Linux 5.9-rc7

There have been some changes to the am335x-<board> DTs related to the
rtc node, so let's re-syncs them with Linux.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210602203805.11494-7-dariobin@libero.it