configs: imx8mm: Define CONFIG_SYS_UBOOT_BASE for i.MX8m

configs: imx8mm: Define CONFIG_SYS_UBOOT_BASE for i.MX8m

The macro `CONFIG_SYS_UBOOT_BASE` is used by SPL loaders `"NOR"` and
`"XIP"` to determine the base address of u-boot.

For `"NOR"` on i.MX8MM it is the base address of QSPI0 plus the offset
of the flattened image tree blob.
Although `QSPI0_AMBA_BASE` is used to define CONFIG_SYS_UBOOT_BASE in
multiple board header files for i.MX8MM, it is not specified.

Specify offset of flattened image tree blob (needs to be set to same
value as specified in 'binman' node), base address of QSPI0 and size of
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>