ppc4xx: Fix problem in 44x cache POST routine

As repoted by Larry Johnson, running "diag run cache" caused a crash
in U-Boot. This problem was introduced by a patch that removed the
TLB entry for the cache test after the test has completed. Since this
TLB was only setup once, a 2nd attempt to run this cache test
failed with a crash. Now this TLB entry is created every time the
routine is called.

Signed-off-by: Stefan Roese <sr@denx.de>