Add SPI controller driver implemented in Socionext UniPhier SoCs.

UniPhier SoCs have two types SPI controllers; SCSSI supports a
single channel, and MCSSI supports multiple channels.
This driver supports SCSSI only.

This controller has 32bit TX/RX FIFO with depth of eight entry,
and supports the SPI master mode only.

This commit is implemented in PIO transfer mode, not DMA transfer.
Signed-off-by: NKunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: NKeiji Hayashibara <hayashibara.keiji@socionext.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NMark Brown <broonie@kernel.org>