arm: hide busdma statistics behind ifdef ARM_BUSDMA_MAPLOAD_STATS

[mjg]

Summary

Stat collection using counter(9) is quite expensive on this platform and
these counters normally are not needed.

In particular we see about 1.5% bump in packet rate using Cortex A9

Sponsored by: Rubicon Communications, LLC ("Netgate")

[• ian]

I suspect these stats aren't much-used in the real world, and can't think of any reason not to do this.

[mjg]

https://cgit.FreeBSD.org/src/commit/?id=aec8ad8a9e6aba2d73c98bd41f2994744aae3d01