ARM: dts: bcm2710-rpi-3-b : Change audio node status as "okay"

Start.elf uses dt overlay concept, so each node status can be
overlayed with dtparam=audio=on in config.txt but u-boot does not.
So, change status of on-board audio dt node as "okay" because dt
overlay concept on u-boot requires too huge effort.

Fixing dts file from arm will also change arm64 because arm64 dts
includes arm dts.

Change-Id: I73b3c807968c031ba046f44e937c6828ee7f0119
Signed-off-by: Hackseung Lee <lhs@dignsys.com>
[jcsing.lee@samsung.com: modify commit message more clearly]
Signed-off-by: Jaechul Lee <jcsing.lee@samsung.com>