pwm-samsung: fix TCMP register setting for 100% duty cycle

The code suggests that -1UL should be written to the TCMP register
in order to achieve a 100% duty cycle, however in practice,
setting the register in this way causes a 0V PWM output.

Nothing in the Exynos4412 manual mentions this magic -1UL value,
so handle 100% duty cycle as TCMP=0, which seems to work for my LED.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Sqwerty: improve LED DT definition

Label not needed - just use node name.

max-brightness not needed - just use Linux default brightness scale
(0-255).

Set to default on (max brightness).
Merge pull request #8 from endlessm/shell/3711

Turn white LED on by default