mfd: rtsx: Make arrays depth and cd_mask static const

Don't populate the arrays depath and cd_mask on the stack but make
them static const.  Makes the object code smaller:

   text    data     bss     dec     hex filename
  25413    7216     448   33077    8135 drivers/mfd/rtsx_pcr.o

   text    data     bss     dec     hex filename
  25151    7360     448   32959    80bf drivers/mfd/rtsx_pcr.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>