mmc: bcm2835-mmc: Support underclocking

Support underclocking of the SD bus using the max-frequency DT property
(which currently has no DT parameter). The sd_overclock parameter
already provides another way to achieve the same thing which should be
equivalent in end result, but it is a bug not to support max-frequency
as well.

See: #2350

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

drivers/mmc/host/bcm2835-mmc.c

@@ -1308,8 +1308,8 @@ static int bcm2835_mmc_add_host(struct bcm2835_host *host)
 
 	host->clk_mul = 0;
 
-	mmc->f_max = host->max_clk;
-	mmc->f_max = host->max_clk;
+	if (!mmc->f_max || mmc->f_max > host->max_clk)
+		mmc->f_max = host->max_clk;
 	mmc->f_min = host->max_clk / SDHCI_MAX_DIV_SPEC_300;
 
 	/* SDHCI_QUIRK_DATA_TIMEOUT_USES_SDCLK */