boards: adi: Fix input subsys tests and samples for max32 boards

Input subystem tests and samples were failing to build on these max32
boards because the zephyr,code property was missing on gpio-keys child
nodes. The devicetree binding doesn't specify the property as required,
but the driver implements a build assert when the property isn't
defined.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>