clocksource: improve sh_cmt clocksource overflow handling

This patch improves the sh_cmt clocksource handling.

Currently the counter value is ignored in the case of
overflow. With this patch the overflow flag is read
before and after reading the counter, removing any
counter value and overflow flag mismatch issues.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>