hw/i2c/mpc_i2c: Fix mmio region size

hw/i2c/mpc_i2c: Fix mmio region size

The last register of this device is at offset 0x14 occupying 8 bits so
to cover it the mmio region needs to be 0x15 bytes long. Also correct
the name of the field storing this register value to match the
register name.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Fixes: 7abb479c7a ("PPC: E500: Add FSL I2C controller")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240721225506.B32704E6039@zero.eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>