ARM: ux500: turn on PRINTK_TIME in u8500_defconfig

I recently noticed slow booting of a board, and without printk timestamps it's
harder to tell just where the delays are coming from. Enable it.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>