'struct cdns_reg_pairs' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  60489	   4342	     16	  64847	   fd4f	drivers/phy/cadence/phy-cadence-sierra.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  60521	   4310	     16	  64847	   fd4f	drivers/phy/cadence/phy-cadence-sierra.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/c57ee8206ebba8c222d7d954335654384072b9ac.1720723132.git.christophe.jaillet@wanadoo.frSigned-off-by: Vinod Koul <vkoul@kernel.org>