spi: add driver for STM32 SPI controller

The STM32 Serial Peripheral Interface (SPI) can be used to communicate
with external devices while using the specific synchronous protocol. It
supports a half-duplex, full-duplex and simplex synchronous, serial
communication with external devices with 4-bit to 16/32-bit per word. It
has two 8x/16x 8-bit embedded Rx and TxFIFOs with DMA capability. It can
operate in master or slave mode.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>