kernel: time: Fix low resolution timer not fire in 32bit case

Low resolution timer is not fired to run at given expired time
which is equal to this cpu's base clk time. This is caused by
32bit intergar overflow.
When pos_up is 0, and pos_down is -1, with unsigned add, it will
not seen pos_up + clk is bigger than pos_down + clk. So add
an cast to u64 to have the expected result.

Change-Id: I45777a1fd282d8f70ba94528b04fce2f0436d7e4
Signed-off-by: Maria Yu <aiquny@codeaurora.org>