dm: timer: Support 64-bit counter

There are timers with a 64-bit counter value but current timer
uclass driver assumes a 32-bit one. Modify timer_get_count()
to ask timer driver to always return a 64-bit counter value,
and provide an inline helper function timer_conv_64() to handle
the 32-bit/64-bit conversion automatically.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/timer/sandbox_timer.c b/drivers/timer/sandbox_timer.c
index 4b20af2..00a9944 100644
--- a/drivers/timer/sandbox_timer.c
+++ b/drivers/timer/sandbox_timer.c
@@ -18,7 +18,7 @@
 	sandbox_timer_offset += offset;
 }
 
-static int sandbox_timer_get_count(struct udevice *dev, unsigned long *count)
+static int sandbox_timer_get_count(struct udevice *dev, u64 *count)
 {
 	*count = os_get_nsec() / 1000 + sandbox_timer_offset * 1000;