ARM: OMAP4: l2x0: Cleanup prefetch offset register setting

The bits [31:5] of Prefetch Offset Register are reserved in
PL310 r2p0 so they should be kept unchanged.
It's enough to set Data prefetch enable bit in PL310 auxiliary
control register to enable the feature, so setting of bit 28 of POR
is removed.
Before writing POR value, the bits [4:0] has to be cleared to remove
previous value.

Change-Id: Id5d047da0daa0c841c69d4d587d2280d2f3433f9
Signed-off-by: Konstantin Shlyakhovoy <x0155534@ti.com>
ARM: OMAP4: l2x0: Set optimized value for prefetch offset

Tuning the value of prefetch offset register of PL310 cache controller
for OMAP4 family. The patch can give up to 4% better results in low level
benchmarks (like bw_mem at block size above 2 Mb) and up to 2% in benchmarks
imitating use cases (like Sunspider).
Note: if double line fill is enabled, the optimal value can be different.

Change-Id: Iba7b7b5950981b851c4eac4ed636ec5d8006c87b
Signed-off-by: Konstantin Shlyakhovoy <x0155534@ti.com>