ARM: dts: bcm283x: Remove gpio line name NC

The convention to name not connected GPIOs with NC has never been
adapted. Also newer Raspberry Pi boards like RPi 4 never did. So fix
this inconsistency by removing all of the NC names.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>