[ARM] tegra: Enable pl310 data prefetching and prefetch offset

[ARM] tegra: Enable pl310 data prefetching and prefetch offset

Enable data prefetching in the L2 cache controller, and set
the prefetch offset to 7.

Memcpy performance measured copying 16 MB buffers 78 times:
Data prefetch disabled, prefetch offset 0: 440 MB/s
Enabling data prefetching, prefetch offset 0: 430 MB/s
Enabling data prefetching, prefetch offset 7: 502 MB/s

Overall, this patch gives a 14% improvement in memcpy performance.

Prefetch offset of 8 causes prefetches to cross 4k boundaries
and cannot be used.

Original-author: Gary King <gking@nvidia.com>
Signed-off-by: Chris Fries <C.Fries@motorola.com>
Signed-off-by: Colin Cross <ccross@android.com>
Change-Id: I7ce0810b3f94edc2640df3f643cf81357052f2f1