ARM: MX5: Fix broken leftover TO-2 errata workaround

This check was broken. r3 does not contain the silicon revision anymore, so
we need to reload it. Also, this errata only applies to i.MX51.

Signed-off-by: David Jander <david@protonic.nl>
Acked-by: Stefano Babic <sbabic@denx.de>