ARM: dts: hummingboard: enable PCF8523 RTC support

Enable the commented out PCF8523 RTC support for Hummingboard pro
base boards.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>