pwm: jz4740: Fix build failure

When commit 9017dc4fbd59 ("pwm: jz4740: Enhance precision in calculation
of duty cycle") from v5.8-rc1 was backported to v5.4.x its dependency on
commit ce1f9cece057 ("pwm: jz4740: Use clocks from TCU driver") was not
noticed which made the pwm-jz4740 driver fail to build.

As ce1f9cece057 depends on still more rework, just backport a small part
of this commit to make the driver build again. (There is no dependency
on the functionality introduced in ce1f9cece057, just the rate variable
is needed.)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>