dt-bindings: leds: leds-bcm63138: unify full stops in descriptions

Description of "reg" doesn't have full stop at the end. It makes sense
as it's a one-sentence only. Use the same style for "active-low".

Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Pavel Machek <pavel@ucw.cz>