dt-bindings: openrisc: Add OpenRISC platform SoC

Add devicetree binding documentation for the OpenRISC platform
opencores,or1ksim.  This is the main OpenRISC reference platform
supporting multiple FPGA SoC's.

This format is based on some of the mips binding docs as we have
similar requirements.

Also, update maintainers so openrisc related binding changes are visible
to the openrisc team.

Acked-by: Rob Herring <robh@kernel.org>
Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Stafford Horne <shorne@gmail.com>