dt/bindings: Add bindings for Micro Crystal rv8803

... and Epson RX8900 real time clock

Signed-off-by: Oleksij Rempel <fixed-term.Oleksij.Rempel@de.bosch.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>