dt-bindings: i2c: npcm7xx: add NPCM I2C controller

Added device tree binding documentation for Nuvoton BMC
NPCM I2C controller.

Signed-off-by: Tali Perry <tali.perry1@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>