SPI: MIPS: lantiq: add FALCON spi driver

SPI: MIPS: lantiq: add FALCON spi driver

The external bus unit (EBU) found on the FALCON SoC has spi emulation that is
designed for serial flash access. This driver has only been tested with m25p80
type chips. The hardware has no support for other types of spi peripherals.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: spi-devel-general@lists.sourceforge.net
Cc: linux-mips@linux-mips.org
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: https://patchwork.linux-mips.org/patch/3844/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>