dt/bindings: Add bindings for the PIC32 real time clock

Document the devicetree bindings for the real time clock found
on Microchip PIC32 class devices.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>