i2c: npcm7xx: Add support for slave mode for Nuvoton

Add support for slave mode for Nuvoton
NPCM BMC I2C controller driver.
Signed-off-by: Tali Perry <tali.perry1@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>