phy: cadence: Sierra: Constify struct cdns_sierra_vals

'struct cdns_sierra_vals' 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
  60521	   4310	     16	  64847	   fd4f	drivers/phy/cadence/phy-cadence-sierra.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  60937	   3894	     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/0ccfd259fdf40475c89cfb22a64c6388e7c646b4.1720723132.git.christophe.jaillet@wanadoo.frSigned-off-by: Vinod Koul <vkoul@kernel.org>