bindings: rtc: correct wrong reference in required properties

There are currently two broken bindings descriptions for RTC:

maxim,ds3231.txt
pcf8563.txt

They broke because of a improper RST documentation conversion with
commit 8c27ceff ("docs: fix locations of several documents that got
moved") and now reference to a non-existing file:

Documentation/devicetree/bindings/i2c/trivial-admin-guide/devices.rst

However, the original reference to i2c/trivial-devices should have never
been made in the first place.

This change fixes this issue by replacing with correct descriptions.

Reported-by: Wolfram Sang <wsa@the-dreams.de>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>