mmc: dw_mmc: remove system PM callback

Now there are no variant drivers using dw_mci_suspend
and dw_mci_resume, so let's remove it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>