sf: Add spi_flash_alloc() to create a new SPI flash struct

At present it is difficult to extend the SPI flash structure since
all devices allocate it themselves, and few of them zero all fields.
Add a new function spi_flash_alloc() which can be used by SPI devices
to perform this allocation, and thus ensure that all devices can
better cope with SPI structure changes.

Signed-off-by: Simon Glass <sjg@chromium.org>