clocksource: dbx500-prcmu: use relaxed readl variant

Modify clksrc_dbx500_prcmu_read to replace readl() with readl_relaxed().
This speeds up calls to the function by about 40%.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>