ARM: dts: aspeed: Use 24MHz fixed clock for pwm

The aspeed pwm driver always sets the clock source to 24MHz, specify
the fixed clock in device tree to make sure the driver is using the
correct clock frequency to calculate the fan speed.

Signed-off-by: Lei YU <mine260309@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>