ppc4xx: Add remove_tlb() function to remove a mem area from TLB setup

ppc4xx: Add remove_tlb() function to remove a mem area from TLB setup

The new function remove_tlb() can be used to remove the TLB's used to
map a specific memory region. This is especially useful for the DDR(2)
setup routines which configure the SDRAM area temporarily as a cached
area (for speedup on auto-calibration and ECC generation) and later
need this area uncached for normal usage.

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