ARM: dts: Modify pwm frequency for APQ8084 liquid

An audible buzzing is heard at the previous
pwm frequency of 10kHz, so raising the frequency
to 20kHz prevents the buzzing.

Change-Id: I9c6f826f9e107641321df2a2c88f4640a7d77d27
Signed-off-by: Casey Piper <cpiper@codeaurora.org>