configs: Add config for enabling FSPI boot option for i.MX8m

configs: Add config for enabling FSPI boot option for i.MX8m

Add imx8mm_evk_fspi_defconfig to build QSPI boot image.
This config is based on imx8mm_evk_defconfig with addtional config options to
define FSPI Header parameters required to generate QSPI Header.
Update SPL offset to include header size  and overwrite IMX_CONFIG to use
lpddr.cfg for FSPI.

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>