Exynos: Add timer_get_us function

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 <[email protected]>
Signed-off-by: Akshay Saraswat <[email protected]>
Acked-by: Simon Glass <[email protected]>
Signed-off-by: Minkyu Kang <[email protected]>