ARM: imx_v6_v7_defconfig: Enable RTC PCF8523 support

RTC PCF8523 is used on some mx6 cubox-i boards.

Let it work by default.

Reported-by: Rick Thomas <rbthomas@cube.rcthomas.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>