tools: mkimage: Add support to generate FlexSPI Header for i.MX8m

tools: mkimage: Add support to generate FlexSPI Header for i.MX8m

Add struct with Flex SPI Configuration Block and enable generating
fspi header using mkimage.

Refer i.MX 8M Mini Application Processor Reference Manual for
detailed information about parameters for FlexSPI Configuration block.

Signed-off-by: Mamta Shukla <mamta.shukla@leica-geosystems.com>
Signed-off-by: Thomas Haemmerle <thomas.haemmerle@leica-geosystems.com>
Tested-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>