mtd: spi: Add lightweight SPI flash stack for SPL

mtd: spi: Add lightweight SPI flash stack for SPL

Add a tiny SPI flash stack that just supports reading data/images from
SPI flash. This is useful for boards that have SPL size constraints and
would need to use SPI flash framework just to read images/data from
flash. There is approximately 1.5 to 2KB savings with this.

Based on prior work of reducing spi flash id table by
Simon Goldschmidt <[email protected]>

Signed-off-by: Vignesh R <[email protected]>
Tested-by: Simon Goldschmidt <[email protected]>
Tested-by: Stefan Roese <[email protected]>
Tested-by: Horatiu Vultur <[email protected]>
Reviewed-by: Jagan Teki <[email protected]>
Tested-by: Jagan Teki <[email protected]> #zynq-microzed