tools: mkimage: Add support for i.MXRT FlexSPI Header

Modify imx8m Flex SPI Configuration Block to work with imxrt.
Add more Flex SPI configuration options to Kconfig.

Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
Tested-by:  Adam Ford <aford173@gmail.com> #imx8mn-beacon