mfd: rsmu: support I2C SMBus access

8a3400x device implements its own reg_read and reg_write,
which only supports I2C bus access. This patch adds support
for SMBus access.

Signed-off-by: Min Li <min.li.xe@renesas.com>
Link: https://lore.kernel.org/r/LV3P220MB12021342F302AADEB6C1601CA0192@LV3P220MB1202.NAMP220.PROD.OUTLOOK.COM


Signed-off-by: Lee Jones <lee@kernel.org>