ARM: dts: Move rpi-otp nodes onto a dedicated bus

The rpi-otp driver uses a virtualised, OTP-relative addressing scheme.
However, when instance nodes are children of "/soc" they appear to be
addressable directly by the host, which is wrong (but not in a way which
causes an error unless one goes looking for one).

Add a wrapper (bus) node without a "ranges" property to make the
separation clear.

See: https://github.com/raspberrypi/linux/issues/6196



Signed-off-by: Phil Elwell <phil@raspberrypi.com>