pwm: keembay: Improve compile coverage by allowing to enable on !ARM64

There are no arm64 specific constructs in this driver and it compiles
just fine with ARCH=arm.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>