dt: lpc32xx: pwm: update documentation of LPC32xx PWM device

NXP LPC32xx SoC has two separate PWM controller devices, update device
tree binding documentation to reflect this fact.

The change makes previous PWM device node description incompatible
with the new version.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>