spi: Add Socionext F_OSPI SPI flash controller driver

Introduce Socionext F_OSPI controller driver. This controller is used to
communicate with slave devices such as SPI flash memories. It supports
4 slave devices and up to 8-bit wide bus, but supports master mode only.

This driver uses spi-mem framework for SPI flash memory access, and
can only operate indirect access mode and single data rate mode.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>