mfd: twl-core: Add twl_i2c_read/write_u16

Add a simple twl_i2c_read/write_u16 wrapper over
the twl_i2c_read/write, which is similar to the
twl_i2c_read/write_u8 wrapper.

Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Tested-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>