exynos: Export timer_get_us() to get microsecond timer

This function, if implemented by the board, provides a microsecond
timer. The granularity may be larger than 1us if hardware does not
support this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rajeshwari S Shinde <rajeshwari.s@samsung.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>