Enable PL310 double linefill for Tuna board

Double linefill was disabled because of single cache line
corruption in first revisions of OMAP 4460. On Tuna board,
after extensive testing, no cache corruption cases were
observed.

Benchmarks (RgBandwidth, Antutu) show up to 45% performance
gains in memory bandwidth tests.

RgBandwidth (Threads: Auto, Test: Misc)

     Single Linefill   Double Linefill

Copy   1403.0 MB/s  ->  1826.6 MB/s
Scale  1386.8 MB/s  ->  1672.2 MB/s
Add    1227.7 MB/s  ->  1778.9 MB/s
Triad  1225.3 MB/s  ->  1746.2 MB/s