iMX28: Fix ARM vector handling

iMX28: Fix ARM vector handling

This patch introduces proper ARM vector handling for i.MX28 CPU. This issue
wasn't addressed because the interrupts weren't enabled on any ARMv5 core,
therefore the issue wasn't noticed earlier.

In previous implementation, the vectoring code used by i.MX28 CPU when an
exception happened was that of the SPL. With this change, the branch target when
an exception happens can be reconfigured by U-Boot.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>