dt-bindings: leds: Add 'cpuX' to 'linux,default-trigger'

Add 'cpu' and 'cpuN' to possible values for 'linux,default-trigger'.
There's 45 cases of them in upstream dts files.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>