mmc: pwrseq: add mmc-pwrseq file to provide a generic interface

Add mmc-pwrseq file to provide a generic interface.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 1c849cb..89d6af3 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -6,6 +6,7 @@
 obj-y += mmc.o
 obj-$(CONFIG_$(SPL_)DM_MMC) += mmc-uclass.o
 obj-$(CONFIG_$(SPL_)MMC_WRITE) += mmc_write.o
+obj-$(CONFIG_MMC_PWRSEQ) += mmc-pwrseq.o
 obj-$(CONFIG_MMC_SDHCI_ADMA_HELPERS) += sdhci-adma.o
 
 ifndef CONFIG_$(SPL_)BLK