ARM: dts: hummingboard: add sgtl5000 support for Hummingboard Pro

Add the DT description for the SGTL5000 found on the Hummingboard Pro
model.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>