Enable d-cache

[tfx2001]

Describe

Since hpm-rs/hpm-rt#11 fixed the d-cache problem when accessing SDRAM, enabling d-cache speeds up SDRAM memory access and improves performance.

Here is ramspeed test result:

~ # ramspeed -b 2 -g 1 -m 1 -r
RAMspeed (GENERIC) v2.6.0 by Rhett M. Hollander and Paul V. Bolotoff, 2002-09

1Gb per pass mode

INTEGER & READING         1 Kb block: 1469.35 Mb/s
INTEGER & READING         2 Kb block: 1485.96 Mb/s
INTEGER & READING         4 Kb block: 1498.95 Mb/s
INTEGER & READING         8 Kb block: 1504.50 Mb/s
INTEGER & READING        16 Kb block: 1506.97 Mb/s
INTEGER & READING        32 Kb block: 1304.37 Mb/s
INTEGER & READING        64 Kb block: 116.81 Mb/s
INTEGER & READING       128 Kb block: 116.84 Mb/s
INTEGER & READING       256 Kb block: 116.90 Mb/s
INTEGER & READING       512 Kb block: 116.84 Mb/s
INTEGER & READING      1024 Kb block: 116.90 Mb/s

Other

Resolves #4