dt: pwm: bcm2835: fix typo in clocks property name

According to the examples of BCM2835 PWM device nodes there is a typo in
'clocks' property name, which is a common property name on clock consumer
side to store a phandle to an input clock.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Rob Herring <robh@kernel.org>