arm64: dts: juno: Fix "debounce-interval" property misspelling

"debounce_interval" was never supported in the bindings. It should be
"debounce-interval". Moreover, latest DTC complains the following:

Warning (property_name_chars_strict): debounce_interval: Character '_' not recommended in property name

This patch fixes the above warning by using the correct property as
per the bindings.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>