mmc: Board-specific MMC power initializations

Some devices may use non-standard combinations of regulators to power MMC:
this allows these devices to provide a board-specific MMC power init function
to set everything up in their own way.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/include/mmc.h b/include/mmc.h
index d74a190..adffc35 100644
--- a/include/mmc.h
+++ b/include/mmc.h
@@ -385,6 +385,7 @@
 int mmc_legacy_init(int verbose);
 #endif
 
+void board_mmc_power_init(void);
 int board_mmc_init(bd_t *bis);
 int cpu_mmc_init(bd_t *bis);
 int mmc_get_env_addr(struct mmc *mmc, int copy, u32 *env_addr);