phy: xgene: make array serdes_reg static const, makes object smaller

Don't populate the array serdes_reg on the stack but instead make it
static const. Makes the object code smaller by 228 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  23875	   6232	     64	  30171	   75db	drivers/phy/phy-xgene.o

After:
   text	   data	    bss	    dec	    hex	filename
  23551	   6328	     64	  29943	   74f7	drivers/phy/phy-xgene.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>