dt-bindings: bus: Add Baikal-T1 APB-bus binding

dt-bindings: bus: Add Baikal-T1 APB-bus binding

Baikal-T1 CPU or DMAC MMIO requests are handled by the AMBA 3 AXI
Interconnect which routes them to the AXI-APB bridge, which in turn
serializes accesses and routes them to the corresponding APB slave device.
This binding describes the AXI-APB bridge considered as the APB-bus. It is
supposed to be compatible with "be,bt1-apb" and "simple-bus" drivers,
should be equipped with EHB MMIO region and a region with no slave device
mapped, interrupts line number, APB reference clock and domain reset line.

Link: https://lore.kernel.org/r/20200526125928.17096-3-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Olof Johansson <olof@lixom.net>
Cc: linux-mips@vger.kernel.org
Cc: soc@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>