metag: cachepart: take into account small cache bits

The CORE_CONFIG2 register has bits to indicate that the data or code
cache is small, i.e. that the size described in the field should be
divided by 64. Take this into account in get_icache_size() and
get_dcache_size().

Signed-off-by: James Hogan <james.hogan@imgtec.com>