spl: Kconfig: Enable SPI_FLASH_TINY by default for SPL

SPL only needs to be able to read from SPI Flash to load next stage and
does not really need write/erase etc. Therefore in order to reduce SPI
Flash code size in SPL, enable SPI_FLASH_TINY, that only supports
reading from SPI flash, as default.

Note: Since, SPI_FLASH_TINY does not support SPI_FLASH_BAR,
SPI_FLASH_TINY is not enabled for boards with SPI controllers that
cannot support 4 byte addressing.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed