Tuna: set PL310 cache controller prefetch offset to 3

Now that double linefill is enabled the PL310 prefetch
offset needs re-evaluation. Benchmarks show that value
of 3 (or 4, close second) is the optimal value.

Performance gains in memory bandwidth tests up to 9%.

RgBandwidth (Threads: Auto, Test: Misc)

Copy   1826.6 MB/s  ->  1915.8 MB/s
Scale  1672.2 MB/s  ->  1812.7 MB/s
Add    1778.9 MB/s  ->  1905.3 MB/s
Triad  1746.2 MB/s  ->  1844.7 MS/s