mmc: dw_mmc-pltfm: include 'dw_mmc-pltfm.h'

Include 'dw_mmc-pltfm.h' header file in order to fix the following
sparse warnings:

drivers/mmc/host/dw_mmc-pltfm.c:36:5: warning: symbol 'dw_mci_pltfm_register' was not declared. Should it be static?
drivers/mmc/host/dw_mmc-pltfm.c:94:1: warning: symbol 'dw_mci_pltfm_pmops' was not declared. Should it be static?
drivers/mmc/host/dw_mmc-pltfm.c:118:5: warning: symbol 'dw_mci_pltfm_remove' was not declared. Should it be static?

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>