kscan: Introduce zephyr,keyboard-scan chosen prop

Introduce zephyr,keyboard-scan chosen node property to point to the
node that implements the default KSCAN device.  This is similar to
the zephyr,display property.

We switch over the samples to utilize the new property instead of
kscan0 alias.

Signed-off-by: Kumar Gala <galak@kernel.org>