keymap: Ignore brightness keys on Dell Inspiron 1520 to avoid double events

On the Dell Inspiron 1520 both the atkbd and acpi-video input devices report
an event for pressing the brightness up / down key-combos, resulting in user
space seeing double events and increasing / decreasing the brightness 2 steps
for each keypress.

This hwdb snippet suppresses the atkbd events, making the Inspiron 1520 work
like most modern laptops which emit brightness up / down events through
acpi-video only.

Reported by Pavel Malyshev <p.malishev@gmail.com>

https://bugzilla.redhat.com/show_bug.cgi?id=1141525

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>