configs: ls1028a: Unset CONFIG_SPI_FLASH_SPANSION

There is no Spansion flash on LS1028A-RDB/QDS boards.
Unset CONFIG_SPI_FLASH_SPANSION option for both the boards.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>