malta: disable L2 caches

Malta boards may be used with cores which support L2 caches, however
U-boot does not yet support L2 cache for MIPS. Thus for the moment we'll
disable L2 caches by setting the L2B bit in Config2. This is specific to
MTI/Imagination MIPS cores which is why this is done for the Malta board
rather than generically.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>