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 <simon.k.r.goldschmidt@gmail.com>

Signed-off-by: Vignesh R <vigneshr@ti.com>
Tested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed