lib: time: Add microsecond timer

Add get_timer_us(), which is useful e.g. when we need higher
precision timestamps.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
[trini: Fixup arch/arm/mach-bcm283x/include/mach/timer.h]
Signed-off-by: Tom Rini <trini@konsulko.com>

include/time.h

@@ -13,6 +13,7 @@ unsigned long get_timer(unsigned long base);
  * Granularity may be larger than 1us if hardware does not support this.
  */
 unsigned long timer_get_us(void);
+uint64_t get_timer_us(uint64_t base);
 
 /*
  * timer_test_add_offset()