iio: make st_sensors drivers use regmap

This patch is meant to replace the i2c/spi transfer functions with
regmap. SPI framework requires DMA safe buffers so let's add GFP_DMA
flag for memory allocation used by bulk_read functions.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>