overlays: pisound: Make button pins owned by card

The pisound overlay claims a number of GPIOs using a pinctrl node, but
for some reason hangs this claim on the main gpio node. This obscures
the ownership of the pins (as shown by gpioinfo and in the kernel logs)
and isn't scalable - only one overlay can use the trick at a time.

Change the overlay to make the pin ownership clear and avoid any future
conflicts, removing a bogus claim on pins_spi0 at the same time.

See: https://github.com/raspberrypi/linux/issues/5235



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