overlays: Rename gpio-fsm property num-soft-gpios

The gpio-fsm property "num-soft-gpios" triggers a kernel DT checker
that warns about the lack of #gpio-cells on a random node with the
phandle that just happens to match the number of soft GPIOs. Rename
the property to "num-swgpios" to avoid the warning.

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