dt-bindings: arm: rockchip: Add FriendlyARM NanoPi M4B

The NanoPi M4B is a minor revision of the original M4.

The differences against the original Nanopi M4 that are common with the
other M4V2 revision include:

  - microphone header removed
  - power button added
  - recovery button added

Additional changes specific to the M4B:

  - USB 3.0 hub removed; board now has 2x USB 3.0 type-A ports and 2x
    USB 2.0 ports
  - ADB toggle switch added; this changes the top USB 3.0 host port to
    a peripheral port
  - Type-C port no longer supports data or PD
  - WiFi/Bluetooth combo chip switched to AP6256, which supports BT 5.0
    but only 1T1R (down from 2T2R) for WiFi

Add a compatible string for the new board revision.

Link: https://lore.kernel.org/r/20210121162321.4538-3-wens@kernel.org
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>