arm: mxs: Fix vectoring table crafting

The vectoring table has to be placed at 0x0, but U-Boot on MX23/MX28
starts from RAM, so the vectoring table at 0x0 is not present. Craft
code that will be placed at 0x0 and will redirect interrupt vectoring
to proper location of the U-Boot in RAM.

Signed-off-by: Marek Vasut <marex@denx.de>
CC: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>