MIPS: Set default pci cache line size.

On MIPS the generic PCI code has always defaulted to L1_CACHE_BYTES
because the architecutre PCI code did not provide a better default.
In particular on systems with S-caches or T-caches this was suboptimal.

Provide a better default by setting pci_dfl_cache_line_size based on
the size of the line size of the lowest level of the cache hierarchy.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/2982/