clk: sunxi-ng: mux: Remove unused 'reg' field

The driver uses 'reg' from 'struct ccu_common' everywhere, so this
duplicate field is unused and unnecessary. Remove it.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210902230336.29745-1-samuel@sholland.org