crypto: hisilicon/hpre - constify struct debugfs_reg32

hpre_cluster_dfx_regs and hpre_com_dfx_regs are never changed and can
therefore be made const.

This allows the compiler to put it in the text section instead of the
data section.

Before:
   text    data     bss     dec     hex filename
  16455    6288     480   23223    5ab7 drivers/crypto/hisilicon/hpre/hpre_main.o

After:
   text    data     bss     dec     hex filename
  16839    5904     480   23223    5ab7 drivers/crypto/hisilicon/hpre/hpre_main.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>