dt-bindings: rtc: Add ST m41t93

Document DT bindings for m41t93 rtc which supports time, date,
alarm, watchdog, square wave clock output provider, user sram
and 8 bit timer.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/b1b1f7abde8a4ee8b625361bca32934679bf1e1a.1758379856.git.akhilesh@ee.iitb.ac.in
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

[ upstream commit: b70598b3769ea64978e76601d1824a12de556da1 ]