Exynos: Add timer_get_us function

timer_get_us returns the time in microseconds since a certain reference
point of history.  However, it does not guarantee to return an accurate
time after a long period; instead, it wraps around (that is, the
reference point is reset to some other point of history) after some
periods. The frequency of wrapping around is about an hour (or 2^32
microseconds).

Test with command "sf probe 1:0; time sf read 40008000 0 1000".
Try with different numbers of bytes and see that sane values are obtained

Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

arch/arm/cpu/armv7/s5p-common/timer.c

@@ -90,6 +90,21 @@ unsigned long get_timer(unsigned long base)
 	return gd->arch.timer_reset_value / 1000 - base;
 }
 
+unsigned long timer_get_us(void)
+{
+	static unsigned long base_time_us;
+
+	struct s5p_timer *const timer =
+		(struct s5p_timer *)samsung_get_base_timer();
+	unsigned long now_downward_us = readl(&timer->tcnto4);
+
+	if (!base_time_us)
+		base_time_us = now_downward_us;
+
+	/* Note that this timer counts downward. */
+	return base_time_us - now_downward_us;
+}
+
 /* delay x useconds */
 void __udelay(unsigned long usec)
 {