input: increase default stack size to 1024

The default 512 bytes stack size is a bit tight for some architecture
and leads to samples running out of stack. Let's default to 1024 and let
the user tweak it down if necessary.

Suggested-by: Benjamin Cabé <benjamin@zephyrproject.org>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>