soc: sunxi: mbus: Only build the driver on ARM/ARM64

This driver exists as a workaround for old devicetrees which are missing
interconnects properties, so it is only useful for those specific
platforms, which all happen to be ARM or ARM64.

This solves the issue that the driver fails to build on RISC-V, where
PHYS_OFFSET is not defined.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220702032520.22129-1-samuel@sholland.org