ARM: bcm2835: fix get_timer() to return ms

Apparently, CONFIG_SYS_HZ must be 1000. Change this, and fix the timer
driver to conform to this.

Have the timer implementation export a custom API get_timer_us() for use
by the BCM2835 MMC API, which needs us resolution for a HW workaround.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
diff --git a/drivers/mmc/bcm2835_sdhci.c b/drivers/mmc/bcm2835_sdhci.c
index b0afc3c..54cfabf 100644
--- a/drivers/mmc/bcm2835_sdhci.c
+++ b/drivers/mmc/bcm2835_sdhci.c
@@ -39,6 +39,7 @@
 #include <common.h>
 #include <malloc.h>
 #include <sdhci.h>
+#include <asm/arch/timer.h>
 
 /* 400KHz is max freq for card ID etc. Use that as min */
 #define MIN_FREQ 400000
@@ -67,11 +68,11 @@
 	 * (Which is just as well - otherwise we'd have to nobble the DMA engine
 	 * too)
 	 */
-	while (get_timer(bcm_host->last_write) < bcm_host->twoticks_delay)
+	while (get_timer_us(bcm_host->last_write) < bcm_host->twoticks_delay)
 		;
 
 	writel(val, host->ioaddr + reg);
-	bcm_host->last_write = get_timer(0);
+	bcm_host->last_write = get_timer_us(0);
 }
 
 static inline u32 bcm2835_sdhci_raw_readl(struct sdhci_host *host, int reg)