ARM: dts: bcm283x: Add missing GPIO line names

The GPIO sysfs is deprecated and disabled in the defconfig files.
So in order to motivate the usage of the new GPIO character device API
add the missing GPIO line names for Raspberry Pi 2 and 3. In the lack
of full schematics i would leave all undocumented pins as unnamed.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>