mmc: increase power up delay

Increase delay for power up in order to support some slower boards.

Also add some comments about why the delays are there.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

drivers/mmc/core/core.c

@@ -460,12 +460,20 @@ static void mmc_power_up(struct mmc_host *host)
 	host->ios.timing = MMC_TIMING_LEGACY;
 	mmc_set_ios(host);
 
-	mmc_delay(1);
+	/*
+	 * This delay should be sufficient to allow the power supply
+	 * to reach the minimum voltage.
+	 */
+	mmc_delay(2);
 
 	host->ios.clock = host->f_min;
 	host->ios.power_mode = MMC_POWER_ON;
 	mmc_set_ios(host);
 
+	/*
+	 * This delay must be at least 74 clock sizes, or 1 ms, or the
+	 * time required to reach a stable voltage.
+	 */
 	mmc_delay(2);
 }