mips32: detect L1 cache sizes if they're not defined

For boards such as the MIPS Malta with an FPGA core card it is desirable
to be able to detect the L1 cache sizes at runtime, since they are not
dependant upon the board but on the FPGA bitstream in use. This patch
performs that detection when the CONFIG_SYS_[DI]CACHE_SIZE macros are
not defined by the board configuration. In cases where the sizes are
detected this patch also removes the restriction that the I-cache &
D-cache line sizes must be the same, as this is not necessarily true.

If the cache sizes are defined by a configuration then they will be
hardcoded as before, so this patch will not add overhead to such
boards.

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