ARM: dts: socfpga: fine-tune L2 cache configuration

Enable double-linefill and increase prefetch offset, which gives
considerable read performance boost. The following numbers were
obtained using lmbench 3.0 bw_mem tool, for easier comparison, the
numbers are pasted in two columns. The test machine has Cyclone V
SoC running at 800MHz MPU clock and 512MiB 333MHz 16bit DDR3 DRAM.

Without patch   | With patch
$ for i in rd wr rdwr cp fwr frd fcp bzero bcopy ; do echo $i ; bw_mem 64M $i ; done
rd              | rd
64.00 526.46    | 64.00 1151.06
wr              | wr
64.00 329.95    | 64.00 346.14
rdwr            | rdwr
64.00 342.07    | 64.00 367.24
cp              | cp
64.00 239.79    | 64.00 322.47
fwr             | fwr
64.00 1027.90   | 64.00 1025.38
frd             | frd
64.00 322.36    | 64.00 641.89
fcp             | fcp
64.00 256.99    | 64.00 408.41
bzero           | bzero
64.00 1028.43   | 64.00 1025.07
bcopy           | bcopy
64.00 294.73    | 64.00 357.19

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>