mips74k: use cache-writeback for memory, not writethrough.

When I ported this code from netbsd I was .. slightly mips74k greener.
I used writethrough because (a) it's what netbsd did, and (b) if I used
writethrough then things "didn't work."

Fast-forward a couple years, more MIPS hacking and a whole lot more
understanding of the bus APIs (the last few commits notwithstanding;
it's been a long week, ok?) and I have this working for arge,
argemdio, spi and ath. Hans has it working for USB. The ath barrier
code will come in a later commit.

This gets the routing throughput up from 220mbit -> 337mbit.
I'm sure the bridging throughput will be similarly improved.

Tested:

• QCA955x SoC, routing workload.