spl: spi: Move the generic SPI loader into common/spl

All the other SPL loaders are in this directory, so move the SPI one in
there too.

There are two board-specific SPI loaders (fsl and sunxi). These remain in
the drivers/mtd/spi directory, since they do not contain generic code.

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