pinctrl: sunxi: dt: do not accumulate npins across probe attempts

init_pins_table() adds to npins in the driver's static descriptor, so
after a probe deferral the count doubles and the next probe fails with
"pin 0 already registered". Reset it first.

Signed-off-by: Pablo Mazzini <pmazzini@gmail.com>
Fixes: f5e2cd34b12f ("pinctrl: sunxi: allow reading mux values from DT")
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>