iio: imu: st_lsm6dsx: introduce locked read/write utility routines

Add st_lsm6dsx_update_bits_locked, st_lsm6dsx_read_locked and
st_lsm6dsx_write_locked utility routines in order to guarantee
the bus access is atomic respect to reg page configuration.
This is a preliminary patch to add i2c sensor hub support since
i2c master registers are accessed through a reg page multiplexer

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>