mmc: dw_mmc: hi3798cv200: make array degrees static const, makes object smaller

Don't populate the array degrees on the stack but instead make it
static const. Makes the object code smaller by 46 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
   5356	   1560	      0	   6916	   1b04	dw_mmc-hi3798cv200.o

After:
   text	   data	    bss	    dec	    hex	filename
   5214	   1656	      0	   6870	   1ad6	dw_mmc-hi3798cv200.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>