spi: Move freescale-specific code into a private header

At present there are two SPI functions only used by freescale which are
defined in the spi_flash.h header. One function name matches an existing
generic SPL function.

Move these into a private header to avoid confusion.

Arcturus looks like it does not actually support SPI, so drop the SPI code
from that board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>