mxssb: Add example configuration files

Add example configuration files for loading U-Boot and it's SPL
on both MX23 and MX28.
Signed-off-by: Marek Vasut <marex@denx.de>

mxsimage.mx23.cfg

+SECTION 0x0
+ TAG LAST
+ LOAD     0x0        spl/u-boot-spl.bin
+ CALL     0x14       0x0
+ LOAD     0x40000100 u-boot.bin
+ CALL     0x40000100 0x0

mxsimage.mx28.cfg

+SECTION 0x0
+ TAG LAST
+ LOAD     0x0        spl/u-boot-spl.bin
+ LOAD IVT 0x8000     0x14
+ CALL HAB 0x8000     0x0
+ LOAD     0x40000100 u-boot.bin
+ LOAD IVT 0x8000     0x40000100
+ CALL HAB 0x8000     0x0