crypto: hisilicon/sec2 - constify sec_dfx_regs

sec_dfx_regs is 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
  17982    7312     480   25774    64ae drivers/crypto/hisilicon/sec2/sec_main.o

After:
   text    data     bss     dec     hex filename
  18366    6928     480   25774    64ae drivers/crypto/hisilicon/sec2/sec_main.o

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