gpio: dwapb: use dwapb_read instead of readl_relaxed

Commit 67809b97 ("GPIO: gpio-dwapb: Change readl&writel to
dwapb_read&dwapb_write") missed this readl_relaxed() usage, I'm not
sure the reason, maybe for performance reason? But if we do care
the performance, we could use the relaxed io in dwapb_read and
dwapb_write.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>